recovery at chiŞinȂu wastewater treatment plant323687/fulltext01.pdf · school of technology and...

ENERGY RECOVERY AT CHIŞINȂU WASTEWATER TREATMENT PLANT

Bachelor Degree Project in Mechanical Engineering ECTS 30 hp

Spring term 2010

Daniel Graan Rasmus Bäckman

Supervisor: M.Sc. Tomas Walander Examiner: Ph.D. Thomas Carlberger

Daniel Graan i

Rasmus Bäckman

Energy recovery at Chişinȃu wastewater treatment plant

Daniel Graan

Rasmus Bäckman

School of Technology and Society

University of Skövde, Skövde, Sweden

Email: b07dangr[at]student.his.se

Email: a07rasmo[at]student.his.se

Examiner: Thomas Carlberger

Supervisor: Tomas Walander

Last modified: 10 June 2010

Abstract Possibilities for energy recovery from sludge at Chişinȃu wastewater treatment plant have been

investigated and evaluated. One way of recovering energy from sludge is to produce biogas through

anaerobic digestion. Which method of biogas usage that is to prefer in Chişinȃu has been evaluated

from a cost-efficiency point of view. There is a possibility that a new waste incineration plant will be

built next to the wastewater treatment plant, and therefore solutions that benefit from a co-operation

have been discussed. The results show that biogas production would be suitable and profitable in a

long time perspective if the gas is used for combined heat and power production. Though, the rather

high, economical interest rates in Moldova are an obstacle for profitability.

Keywords: Energy recovery, Biogas, Anaerobic digestion, Sludge management, Chisinau,

Kishinev.

School of Technology and Society Energy recovery at Chişinȃu wastewater treatment plant

University of Skövde

Daniel Graan ii

Rasmus Bäckman

Table of contents 1 Introduction ......................................................................................................................... 1

1.2 Problem definition ........................................................................................................................................ 1

1.3 Aim........................................................................................................................................................................ 1

1.4 Boundaries ........................................................................................................................................................ 1

2 Present situation ................................................................................................................. 1

2.1 Chişinȃu .............................................................................................................................................................. 1

2.2 APA Canal .......................................................................................................................................................... 1

2.3 Wastewater treatment plant .................................................................................................................... 1

2.4 Waste management in Chişinȃu ............................................................................................................. 3

2.5 Currencies ......................................................................................................................................................... 4

2.6 Interest rate ...................................................................................................................................................... 4

3 Theory: Anaerobic digestion ........................................................................................... 4

3.1 The chemical process .................................................................................................................................. 4

3.2 pH value ............................................................................................................................................................. 5

3.3 Temperature .................................................................................................................................................... 5

3.4 Retention time ................................................................................................................................................ 6

3.5 Hydrogen Sulphide ....................................................................................................................................... 6

3.6 Construction of an anaerobic reactor .................................................................................................. 6

3.7 Swedish example of an anaerobic digester ....................................................................................... 7

4 Theory: Usage of biogas .................................................................................................... 7

4.1 Flares ................................................................................................................................................................... 7

4.2 Heat production ............................................................................................................................................. 7

4.3 Electricity production.................................................................................................................................. 7

4.4 Sell biogas to waste incineration plant ............................................................................................... 8

4.5 Upgrading of biogas ...................................................................................................................................... 8

4.6 Using excess heat for cooling purposes .............................................................................................. 9

5 Theory: Incinerate sludge .............................................................................................. 10

6 Theory: Investment calculations .................................................................................. 10

7 Solutions for energy recovery ....................................................................................... 10

7.1 Keep the present solution ...................................................................................................................... 10

7.2 Produce biogas in an anaerobic digester ........................................................................................ 11

7.3 Heat boiler...................................................................................................................................................... 11

7.4 CPH plant from ELTECO (present engines) ................................................................................... 11

7.5 CHP plant from MWM ............................................................................................................................... 12

7.6 CHP plant from GE Jenbacher ............................................................................................................... 12

7.7 CHP plant with Turbine ........................................................................................................................... 12

7.8 Upgrading of biogas ................................................................................................................................... 13



Daniel Graan iii

Rasmus Bäckman

7.9 Fuel cell............................................................................................................................................................ 13

7.10 Sell biogas to a waste incineration plant ......................................................................................... 13

7.11 Incinerate sludge ........................................................................................................................................ 14

8 Calculated potential amount of biogas ....................................................................... 14

8.1 Method ............................................................................................................................................................. 14

8.2 Results .............................................................................................................................................................. 15

9 Results of economical calculations .............................................................................. 16

9.1 Heat boiler...................................................................................................................................................... 16

9.2 CPH plant from ELTECO (present engines) ................................................................................... 16

9.3 CHP plant from MWM ............................................................................................................................... 16

9.4 CHP plant from GE Jenbacher ............................................................................................................... 17

9.5 CHP plant with Turbine ........................................................................................................................... 17

9.6 Upgrading of biogas ................................................................................................................................... 17

9.7 Sell biogas to a waste incineration plant ......................................................................................... 18

9.8 Incinerate sludge ........................................................................................................................................ 18

10 Conclusions ......................................................................................................................... 18

11 Discussion ........................................................................................................................... 19

11.1 Profits of an investment that is hard to measure in money ................................................... 19

11.2 Energy prices ................................................................................................................................................ 19

12 Future work ........................................................................................................................ 19

13 Acknowledgments ............................................................................................................ 20

14 References .......................................................................................................................... 20



Daniel Graan 1 of 21

Rasmus Bäckman

1 Introduction

1.2 Problem definition

The wastewater treatment plant (WWTP) in the Moldovan capital, Chişinȃu, has problems

handling the sludge from the cleaning processes. The lack of stabilization of the sludge results in a

larger amount of sludge than necessary and that an odour sometimes is spread over the city. It also

causes uncontrolled methane production that results in greenhouse emissions. Sludge contains energy,

which at the moment is not taken advantage of.

1.3 Aim

The project aims to investigate various techniques for energy recovery and handling of sludge. To

make this possible, the energy content of the sludge must be calculated. The calculated potential

energy gain is to be compared to municipal energy needs in areas such as public transport, heating and

cooling. The main technique for energy extracting is anaerobic digestion, which both decreases the

amount of sludge and takes away the odour. This also gives the opportunity to extract renewable and

carbon neutral energy in form of biogas. This report will present possible biogas production methods

and different uses of the biogas that can be produced.

1.4 Boundaries

The project will:

• explore various technical options for gas usage and compare which is the most cost-effective.

• propose a method for extracting biogas from the sludge.

• calculate the lost economical value of not digesting sewage sludge.

• not focus on the sewage treatment process.

• not analyze the metals and toxins content of sludge.

2 Present situation

2.1 Chişinȃu

Chişinȃu is the capital city of Moldova and has 598400 inhabitants 2008 (1)

. The city as well as the

rest of Moldova was a part of the Soviet Union until 1991, when the Soviet Union fell apart. A lot of

investments that were made during the Soviet time are still used today. An example of this is the gas

grid and a central heating system which provides the whole of the city with heat. For public

transportation the city traffic network is connected by trolleybuses. Some busses are also powered by

diesel engines which make their routes independent of the electrical grid used for trolleybuses.

2.2 APA Canal

The company in charge of the water treatment in Chişinȃu is APA Canal, a municipal company

which was founded 1892. APA Canal both does water treatment and wastewater treatment. They also

maintain the pipe network. A minor part of the central heating and warm water in Chişinȃu is also

provided by APA Canal Chişinȃu.

2.3 Wastewater treatment plant

There is only one WWTP in Chişinȃu and major parts of the plant are built 1972. The volume of

incoming sewage water is about 150´000 to 170’000 m³/day (2)

.




Rasmus Bäckman

Figure 1. An overview of Chişinȃu WWTP. “Statia de epurare biologica” means Biological WWTP.

2.3.1 The waste water treatment process (2) (3)

The first that is done with the incoming water is to sort out all larger pieces that the process later on

cannot handle, things that should not have been put into the sewage from the very beginning. The

second stage in the treatment process is a sand trap (A in Figure 1) which sorts out sand from the

wastewater. After the sand trap there is a primary clarifying process which takes place in several

sedimentation tanks (B in Figure 1), in which primary sludge is separated from the water. The amount

of sludge is normally about 1000 m³/day. The sludge has a dry substance (TS) of about 5 %. The water

proceeds to the aeration tanks (C in Figure 1) where the wastewater is biologically treated. The second

process with sedimentation tanks (D in Figure 1) is the last step, where secondary sludge (also called

bio sludge) is separated from the water. This sludge contains bacteria that are needed for the treatment

process and the sludge is therefore taken back in to the process by blending it with the incoming

wastewater in to the WWTP.

2.3.1.1 Sludge management (2) (3)

The solution to handle primary sludge is since December 2009 to add two polymers named

Derwfloc 460 and Floerger FR7670 to the sludge. After adding the polymer, the sludge is put in to

tubes made of a rubberlike material. These tubes are provided by a Dutch company named Tencate and

are sold under the trade name Geotube. The sludge stays in these tubes for about 30 days. These tubes

are used only for dewatering of sludge. The tubes increase the dry substance from (TS) 5 % up to 20-

25 %. When the dewatering process is finished the tubes are cut apart and the sludge is stored in a dry

bed for several months before it is used in agriculture as fertiliser for products which has no contact

with the food industry. Examples of such products are wood and flowers. A problem is that anaerobic

digestion sometimes takes place in the dry beds causing the greenhouse gas methane to escape into the

atmosphere.




Rasmus Bäckman

Figure 2. Dewatering device named Geotube.

2.3.2 Operational cost

The wastewater treatment plant has an energy demand (2)

of 16’000 MWh/a (where “a” stands for

annual), about 2MW (2)

. Present price for electricity is 1.330 MDL/kWh (0.079 €/kWh) (4)

. The

treatment of sludge has running costs for the geotubes, polymer and disposal of dried sludge. The

companies that handle disposal of dried sludge are paid 40 MDL/m³ (2.38 €/m³) (3)

.

2.3.3 Anaerobic digesters

The WWTP in Chişinȃu has two anaerobic digesters from 1972 which never have been working

because of leakage problems (2)

.

2.3.4 Gas engines with electricity generators (5)

Next to the anaerobic digesters there is a facility with two gas engines. The facility was installed

somewhere around 2004. The engines are made for both biogas and natural gas. The installation cost

was about 3’500’000 € and the facility was used for 2 years. The facility was shut down because of

increasing prices of natural gas. During usage, only electrical energy was produced and no excess heat

was used for heat production. There was an external company, not APA Canal, who owned the facility.

When the price of natural gas increased, a bank took over the facility and has been the owner since

then. This bank wants to sell the facility for around 20’000’000 MDL (1’189’016 €). The engines and

generators have a total efficiency (electrical and thermal) of 93%, with the capacity of 2*960

kWelectricity and 2*1360 kWthermal.

2.4 Waste management in Chişinȃu

2.4.1 Toxic liquid waste (5)

All toxic liquid waste produced by industries in Chişinȃu is handled by the Environmental

Ministry.




Rasmus Bäckman

2.4.2 Non toxic liquid waste (5)

APA Canal is in charge of waste management for all non toxic liquid wastes in Chişinȃu. There is a

national law that oblige all companies with industrial wastewater to treat their wastewater by

themselves. Around the year 1990 there were about 125 companies that had facilities for wastewater

treatment, but now in 2010 there are only 2-3 of these that are still working. If companies with

untreated wastewater make a contract with APA Canal they are, for a fee, allowed to emit their waste.

There are about 500 companies that have contracts like this with APA Canal and 300 of them are in

food business (restaurants, slaughterhouses, etc.). These contracts make it legal to put untreated

wastewater in to the normal sewage grid where grease causes major problems by plugging pipes,

especially at sections where the slope of the pipe is low. Plugged pipes causes maintenance costs for

APA Canal. Grease also causes a cost in the WWTP by increasing the organic load in the wastewater.

2.4.3 Non liquid waste (6)

The Auto Sanitation Department is in charge of dry waste in Chişinȃu. All waste is sorted and

waste that is not possible to recycle is put on a landfill 35 km outside of Chişinȃu. The Auto Sanitation

Department investigates other solutions for taking care of the waste and they plan to build an

incineration plant. The location of this plant is already decided to be next to the present WWTP.

2.5 Currencies

All prices in this report are written in Euro (€) and prices that are given in Swedish Kronor (SEK)

and Moldovan Lei (MDL) are converted into Euro (€). The exchange rates used in this report are:

1 SEK = 0.104019 € (7)

1 MDL = 0.0594508 € (8)

1 USD = 0.787910 € (9)

2.6 Interest rate

In Moldova the interest rates are around 18 % (10)

, which is very high compared to an average

European value of 5 %. This is a problem, because it limits the possibilities to do profitable

investments. In this report used values for interest rate are 18 % and 5 %. This is to show the

profitability of different investments.

3 Theory: Anaerobic digestion Anaerobic digestion takes place in absence of free oxygen, in comparison with aerobic digestion

which demands free oxygen. Anaerobic digestion is performed by several different anaerobic bacteria.

3.1 The chemical process

The chemical process of anaerobic digestion could be divided into three main stages. In the

chemical process biological substrates such as manure, sludge, food leftovers and grain are digested.

3.1.1 Hydrolysis Stage

The substrate consists of major complex parts such as protein, carbonates and fats. To be able to

digest such pieces these must be broken down into simpler pieces. This is done by hydrolytic bacteria.




Rasmus Bäckman

3.1.2 Acid-Forming Stage

After the hydrolysis stage, other bacteria are able to handle the simpler compounds and produce

different acids. In this stage hydrogen and carbon dioxide are also produced. The most important acid

produced is acetate which has a major role in the following methane production.

3.1.3 Methane producing Stage

In the final stage, the methane forming bacteria are converting carbon dioxide, acetate and

hydrogen into methane. This stage can only handle certain chemical compounds and is therefore

dependent on the previous stages. These three stages must be balanced in a way that none of them

inhibits the process which would bring imbalance to the anaerobic digester. The methane forming

bacteria is sensitive and a decrease in methane production is therefore an early indicator that

something starts to go wrong in the reactor.

3.1.4 Nutrients

The anaerobic process needs different nutrients to operate and as for many other environments

balance is of great importance. To be able to handle organic waste, consisting of carbon, major

amounts of nitrogen is necessary to continue the anaerobic digestion. Nitrogen is also a risk for the

process because high levels of nitrogen lead to ammonia (NH3) production, which is toxic for the

methane forming bacteria. Also phosphorus is an important nutrition for anaerobic digestion. One way

of monitoring the balance between nutrients is to measure the ratio between chemical oxygen demand

(COD), nitrogen (N), phosphorus (P) and sulphur (S). Deublin and Steinhauser claim that a balanced

anaerobic process has an organic matter ratio of COD:N:P:S=800:5:1:0.5 (11)

. Gerardi claims that

generally a ratio of organic matter is COD:N:P =1000:7:1 to COD:N:P=350:7:1 (12)

.

3.2 pH value

A parameter that is important for the anaerobic digestion is pH. Most of the methane forming

bacteria has an optimum pH of 6.8-7.2 (12)

. To keep this pH, alkalinity is of great importance because

the hydrolysis stage produces acids that decrease the pH. The methane forming process counteracts

this decrease in pH. Alkalinity works as a buffer that will ensure that no rapid changes of pH take

place. It is therefore important to measure the alkalinity to see that no sudden decrease can take place.

This is also an early indicator that something is wrong in the digestion and interventions are needed.

3.3 Temperature

There are mainly two different intervals of temperature that are optimal for anaerobic digestion.

Anaerobic digestion can occur outside these intervals, but the optimal is within. The digestion itself

does not generate heat as an aerobic digestion would do so the reactor has to be heated.

3.3.1 Mesophilic digestion

The first interval is called mesophilic digestion and takes place between 32-42 °C (11)

. There are

also several examples of reactors that use 37 °C. It is important for the process to keep the same

temperature to avoid that the bacteria die. An advantage of mesophilic digestion is that there is several

different methane forming bacteria that operate within this temperature interval, which makes the

process less vulnerable.

3.3.2 Thermophilic digestion

The second interval is called thermophilic digestion and these bacteria have their ideal living

temperature about 48-55 °C (11)

. This technique demands more heat energy and the bacteria are also

more sensitive to toxicants and change in temperature. A benefit from this process is that the high

temperature during a longer time removes pathogens that could be within the substrate.




Rasmus Bäckman

3.4 Retention time

An important parameter in anaerobic digestion is the retention time of substrate inside the digester.

The retention time is the average time substrate stays inside the digester, and because the continuous

mixing of substrate it is only possible to calculate an average value for retention time. Deublin and

Steinhauser claim that retention time in a thermophilic digester is 10 days (11)

and 30 days for a

mesophilic digestion (11)

. Svenskt Vatten suggests a retention time of 10 to 20 days (13)

for digesting

sludge at 37 °C depending on chosen teqnique. A problem with short retention time is that the grade of

decomposition could be low and that the wash-out of methane-forming bacteria could be greater than

the bacterial growth. Therefore, Gerardi claims that shorter retention time than 10 days is not

recommended (12)

.

3.5 Hydrogen Sulphide

Hydrogen sulphide (H2S) is a gas that is produced in anaerobic digestion. This gas has a most

unpleasant odour and it is strongly corrosive to different materials because of its low pH value. This

makes hydrogen sulphide unsuitable to all types of facilities, since it affects pipes, tanks and also

engines in a negative way.

3.5.1 Hydrogen Sulphide reduction (11)

The negative properties of hydrogen sulphide make it important to decrease the amount as much as

possible. One way to handle this problem is to use biological methods based on microorganism that

feed on hydrogen sulphide. There are several ways to use these microorganisms. The simplest method

is to let the organisms grow on a surface within the reactor. This can be done by hanging plates of

cloths from the ceiling or use the inner walls, above the surface of the sludge, as suitable

environments. These microorganisms need oxygen but since the whole point of anaerobic digester is

absence of oxygen a well adjusted level of oxygen needs to be added next to the surface where the

micro organism are attached. This method is recommended only for small reactors.

For bigger reactors the gas could be cleaned by a trickling filter outside the reactor. The gas pass

through a filter where microorganisms are able to handle the hydrogen sulphide.

Another external cleaning method is called a Bioscrubber where biogas reacts with caustic soda

(NaOH) to remove the hydrogen sulphide.

One solution that differs from the above mentioned is to prevent hydrogen sulphide from forming

in the process. This can be done by binding sulphur into iron sulphide by adding iron(II) chloride

(FeCl2) or iron(III) chloride (FeCl3) to the sludge before it enters the reactor. The iron sulphide will

then leave the reactor in the outgoing residue. This method has high operational cost but it is a simple

method and gives a good result, less than 150 mg H2S/Nm3

biogas (where Nm3 stands for “normal cubic

meter” and is defined as one cubic meter at a pressure of 1.01325 bar and a temperature of 0 ⁰C).

It is the further use of the biogas that determines how low the amount of hydrogen sulphide must

be. This could be limits given from manufactures of engines or national demands of the quality of gas

entering the gas grid etc.

3.6 Construction of an anaerobic reactor (11) (12)

The most basic anaerobic reactor consists of organic substrate placed in a tube with a sealed lid on

top of it. In that tube biogas will be extracted. But even though it is based on the simple theory of a

closed tube, more advanced facilities are required to be able to handle major amounts of substrate.

When the microorganisms in the anaerobic processes start to work the substrate divides into different

layers. The upper scum layer blocks the gas from seeping out and the pressure from the produced gas

can then cause a flood in the reactor. To avoid this, the substrate is stirred. The most common

technique for stirring is to use some kind propeller, but it is also possible to lead back extracted biogas

to the bottom of the reactor and let it flow though the substrate. It is also important to keep a constant

and convenient temperature in the reactor to maintain a good environment for the microorganisms. To

create this environment the reactor is insulated and incoming sludge is heated.




Rasmus Bäckman

There are different examples of how a biogas plant is planned. The most common technique uses

only one step of digestion in which all the processes take place (hydrolysis, acid production and

methane production). This is either made in one single reactor or several reactors that are connected in

parallel.

3.7 Swedish example of an anaerobic digester

The WWTP of the Swedish city Gothenburg, similar in size to Chişinȃu, uses anaerobic digestion

for sludge management. To be able to compare these cities it is important to notice that they have

about the same annual average temperature (14) (15)

. The WWTP in Gothenburg is run by the company

Gryaab and it produces about the same amount of sludge as the WWTP of Chişinȃu (16) (17)

. Gryaab has

two parallel anaerobic digesters for processing sludge. The size of each anaerobic digester is 11’400

m3 (16)

. They operate at 37 °C (16)

and have a retention time of about 20 days (17)

. About 200 m3 in the

top of the digester’s volume is filled with biogas (17)

, which is equal to about 0.88 % of the digester

volume. The energy consumption of the anaerobic digester in Gothenburg is 4’000 MWhel/a and

11’000 MWhthermal/a (17)

. The produced biogas is sold to a local energy company, Göteborgs Energi,

which is upgrading the biogas (18)

and distributes gas e.g. as vehicle fuel.

4 Theory: Usage of biogas Biogas consists of 65-75% methane

(11). The energy content in methane is high quality energy and

is therefore possible to use in many different ways.

4.1 Flares

Methane is a strong greenhouse gas (20 times more harmful than carbon dioxide (19)

) and therefore

it is important not to let biogas into the atmosphere because of its high methane content. It is therefore

better to burn biogas so water vapour and carbon dioxide are released instead. Burning the excess

biogas is a good emergency solution.

4.2 Heat production

To produce heat from a high quality energy source may give a high efficiency. Heat production, in

its simplest form, is an ancient technique and it is effective. The problem though is to find customers

that are willing to pay for heat over the whole year. It is also important to discuss whether it is right

from an environmental perspective to convert high quality energy directly to low quality energy (20)

.

4.2.1 Gas Boiler (21) (22)

The gas boiler is a device for heating houses and facilities. In relation to biogas production it is also

used for providing the bioreactors with heat to maintain the process of the microorganisms. In the gas

boiler the gas is burned in a combustion chamber where heat is transferred via a pipe. A liquid medium

is transferring the heat through the pipe to radiators. The returning liquid medium has a lower

temperature and flows back to the combustion chamber to get reheated. Newer gas boilers, called

condensing boilers, lead the liquid medium through a heat exchanger. This takes care of the heat in the

exhaust gas to increase the degree of efficiency.

4.3 Electricity production

The internal demand of electricity at the WWTP makes it interesting to investigate whether it is

possible to produce electricity to a lower price than it costs to buy on the energy market. A common

disadvantage for all electricity production techniques is production of excess heat. In order to increase

the total efficiency of power production it is beneficial to find an application that takes advantage of

excess heat. This can be done in a so called CHP plant (Combined Heat and Power plant).




Rasmus Bäckman

4.3.1 Gas turbine (23) (24)

A gas turbine produces energy out of fuels such as methane. The fuel is mixed with air and

compressed before combusted. After the combustion chamber the exhaust gas passes through a turbine

under very high pressure. The turbine is connected to a generator which produces electricity. The

efficiency is about 30% for electricity production from chemical energy in the fuel. A gas turbine

could be combined with a heat exchanger to take care of excess energy.

4.3.2 Gas engine (25)

A gas engine is a traditional piston engine. The fuel is combusted inside the cylinder and the

pressure transfers energy to the rotating crank shaft. Gas can be combined with diesel in some types of

engines. The efficiency for electricity production is about 30-40%. If the cooling system is constructed

in a way that thermal energy could be extracted it is possible to get a total efficiency of 90%.

4.3.3 Fuel cell (26) (27)

Fuel cell is a technology that is under research. It uses electrolyte which is a sort of cold

combustion were fuel is oxidized. A fuel cell has one anode (1 in Figure 3) and one cathode (2 in

Figure 3). These are separated by a membrane which allows ions to pass through, which leads to

potential difference between the anode and cathode. The potential difference makes the electrons from

the anode to strive to reach the cathode. This strive can be used to create an electrical circuit. The

efficiency of a fuel cell today is 30-40% but it will probably be higher in the future when research will

results in new techniques.

Figure 3. The figure is an example of the theory behind fuel cells, here driven by hydrogen gas.

4.4 Sell biogas to waste incineration plant

If there will be a waste incineration plant next to the WWTP (6)

, there is a theoretical possibility to

incinerate biogas in the waste incineration plant together with waste (28)

. To be able to sell biogas to a

future waste incineration plant, a very close co-operation has to be achieved between the involved

companies. In the co-operation, the WWTP has to be able to buy heat for the anaerobic digester in a

beneficial way.

4.5 Upgrading of biogas

The major difference between biogas and natural gas is the methane ratio, which for biogas

extracted from sewage is 60-65 % (12)

. To be able to sell biogas as an alternative to natural gas the

quality of the gas has to obtain the same standards. This can be achieved by upgrading the biogas. For

this there are several techniques.

4.5.1 Absorption

One group of techniques is based on the possibility to have a medium that absorb carbon dioxide

but does not affect the methane.




Rasmus Bäckman

4.5.1.1 Using water as absorption liquid (11)

When water is used as the absorption liquid, water at a temperature of 5-25 ˚C is kept in a column

under pressure and biogas at a pressure of 10-12 Bar is added in the bottom of the column. The carbon

dioxide is chemically bonded to the water but the methane passes through to the top of the column.

4.5.1.2 Chemical absorption (11)

Chemical absorption works similar to water absorption but uses chemicals as the absorption liquid.

These chemicals can be based on glycol or ethanolamines.

4.5.2 Adsorption (11)

One technique for upgrading biogas is Pressure Swing Adsorption (PSA) which uses molecular

sieves. These sieves can be made of activated charcoal, zeolite or carbon. The sieves are placed inside

columns in which the biogas is added under pressure and the carbon dioxide is adsorbed on the sieves

resulting in upgraded biogas. With Pressure Swing Adsorption it is possible to reach very high

methane ratios, up to 99%.

4.6 Using excess heat for cooling purposes

During cold periods of the year there is a market demand for heat and it is therefore possible to sell

excess energy. During warmer periods the request for heat is lower. It could therefore be of great

interest to be able to produce cooling instead.

4.6.1 Central Cooling

Central cooling is a way of providing households and factories with cold. A central cooling facility

produces and transmits the cold through pipelines to customers. In each house or factory the cold

carrying liquid that flows through the pipes is led through radiators. Because the liquid holds a lower

temperature than the air in the house it absorbs heat. The liquid is then transported back to the central

cooling facility to once again get cooled.

4.6.2 Different Cooling techniques (29)

4.6.2.1 External cooling source

An external cooling source can be e.g. water from a sea or a river, where cold is produced from by

a heat exchanger.

4.6.2.2 Refrigerant machines

Refrigerant machines work like a refrigerator, but are a lot bigger than those for domestic use. Its

efficiency is depending on the temperature of the air outside. This is a disadvantage because the

efficiency decreases and the demand of cooling increases when the outside temperature rises, though it

is possible also to take advantage of e.g. cleaned water from a wastewater treatment plant, which has a

lower temperature than the air during summer.

4.6.2.3 Accumulated cold

An accumulator has the ability to store energy in some way. In central heating, large accumulator

tanks are used to store major amounts of cold water when the supply is good to be used later on when

the demand is larger. It is also possible to use an underground lake as an accumulator.

4.6.2.4 Absorption cold

The absorption cold is produced similarly to a compressor fridge, but instead of a compressor, an

absorber, a circulation pump and a generator are used. The required energy is supplied by heat from

e.g. a factory. The efficiency is not that high so it fits best where a lot of waste heat is available.




Rasmus Bäckman

5 Theory: Incinerate sludge Incinerating sludge costs a lot of energy because of the low dry substance (TS) of sludge. There are

two possible methods, either incinerating pure sludge or mixing it with an additive energy source. To

be able to incinerate pure sludge, it must be dried before incineration, first mechanically and then

thermally. Incineration is mainly a method to get rid of sludge because the energy content in sludge is

almost equal to the required energy for drying. In an ultimate situation a small amount of low quality

energy can be gained from the process (13)

.

6 Theory: Investment calculations A method to see if an investment is feasible is to calculate the payback period (T). It is a very

simple method and does not consider how the interest rate affects the investment. The method simply

divides the investment cost (G) with the annual profit (A). The payback period is useful only if the

payback period is short.

Equation 1. An equation for calculation the payback period. (30) (31)

To calculate with the interest rate (r), it is possible to use Present value factor (PVF) which is a

factor of a sequence of yearly payments during a given number of years (n).

Equation 2. An equation for calculating the present value factor. (30)

If the PVF is multiplied by the annual profit (A) it will result in the value of what the sum of profits

would be worth today, the Present value. When this sum is equal to the investment cost (G), payback

of the investment is reached.

Equation 3. An equation where the investment cost is equal to the Present value of yearly profits. (30)

Equation 3 can be rewritten so it is possible to determine the number of years (n) with the profit (A)

it takes to cover the investment cost (G) with the interest rate (r) included:

Equation 4. An equation for calculating the number of years before an investment has reached payback.

7 Solutions for energy recovery

7.1 Keep the present solution

For comparison, the present situation is regarded as reference: the sludge is not stabilized and there

is no energy recovery.

7.1.1 Required investments

There is no major investment that has to be done to continue the current sludge management, but

there are running costs for Geotubes and fees for dispatch of the dried sludge. The cost of a Geotube is

rather high (2)

and the fee for disposal of sludge is 40 lei/m3

sludge (2.38 €/m3

sludge).




Rasmus Bäckman

7.1.2 Benefits and comments

A great advantage of this alternative is that there is just the running cost, though the alternative to

keep the present solution brings several concerns. One question is if the running costs are a cheaper

solution in the long term compared to the other alternatives which requires major investments. Another

question that has to be discussed is if it is acceptable not to stabilize the sludge, from an environmental

point of view.

7.2 Produce biogas in an anaerobic digester

An anaerobic digester is a vital condition for all solutions in this report that includes biogas. A

suitable option for biogas production in Chişinȃu would be a facility similar to the one in Gothenburg.

That means to use mesophilic digestion at 37°C and a retention time of 20 days. These numbers are

confirmed as suitable by both Svenskt Vatten (13)

and Deublin and Steinhauser (11)

.


Deublin and Steinhauser claim that the cost of an anaerobic digester is dependent on its volume and

is about 300-500 $/m3 (236-394 €/m

3), where 394 €/m

3 is used for small scale digesters and 236 €/m

3

for a large scale digester. Chişinȃu is here seen as a large scale facility. The size of an anaerobic

digester can be calculated by multiplying the inflow rate volume per day (Vin= 350’000/365= 960

m3/day), the retention time (t= 20 days) and the factor of space inside the digester that is filled with

biogas and also works as a buffer space (f). The factor (f) is chosen to be f= 1.25. If the factor is as

large as 1.25, there is a margin of 25% in case the inflow and/or the retention time are increased.

Equation 5. Required volume of the digester (11).

Equation 6. Calculated investment cost (11).


Biogas production will give APA Canal an opportunity to produce energy. This is a great benefit

but the major investments that have to be done make it uncertain whether it is possible to gain that

much money on the energy production. It is important to consider these calculations as approximate.

Changes in the variables, Vin, t and f will cause corresponding changes in the investment amount.

7.3 Heat boiler

One of the easiest ways of taking care of the energy in biogas is to combust it in a heat boiler. It has

high efficiency and is mostly suitable for heating of facilities.

Even if it is easy to take care of the energy in the biogas there are in this case some major

problems. Despite a high efficiency, the low heat price makes this solution hard to make profitable, but

also the fact that the demand of heat is low during summertime. To show these problems the most

favourable situation has been chosen for this solution: the efficiency is set to 100 % and the investment

cost to 0 €.

7.4 CPH plant from ELTECO (present engines)

A natural alternative to investigate is whether it is possible to use the engines that already exist on

the WWTP. Because they have not been used for some years it must be of interest for the bank that

owns the engines (5)

to sell them to prevent further capital loss.




Rasmus Bäckman


More than just invest in an anaerobic digester, APA Canal has to buy the engines (Petra

1250CDB). The price for the engines is 20’000’000 MDL (1’190’000 €) (5)

. There is no information

available about maintenance cost and therefore a value of 100’000 €/a is estimated in relation to the

two CHP plants MWM TCG 2020 V20 (see section 7.5.1) and JMS 616 GS-B.L (see section 7.6.1).


An important benefit from a CHP is the possibility to produce energy for internal use. No contact

will be needed with external customers because all electricity will be used within the WWTP which

makes it less complicated. A major question is what can be done with the excess heat during summer

time. Calculations of economical sustainability in this solution depend on how much produced thermal

energy it is possible to sell. Possible solutions are to find industries in the region that are willing to buy

heat all year round or there may be a solution where it is possible to use heat for cold production

during the months when that is requested.

7.5 CHP plant from MWM (32)

As the price for electricity is much higher than the gas prices it is desirable to produce as much

electricity of the biogas as possible. One alternative for production of electricity and heat is a CHP

plant consisting of a gas engine with a generator.

7.5.1 Required Investments

Investments required for this alternative are a biogas facility and a gas engine with a generator. The

investment cost is for the chosen CHP plant is about 900’000 €. Maintenance cost is estimated to be

15.6 €/h (137’000 €/a).


The biogas is in no need of being upgraded, though purification from hydrogen sulphide will

decrease the corrosion and therefore increase the lifetime of the engine. The chosen engine is the

MWM TCG 2020 V20 which has an electrical output of 2 MW, an electrical efficiency of 42.0 % and

a thermal efficiency of 43.8 %. This makes a total efficiency of 85.8 %.

7.6 CHP plant from GE Jenbacher (33)


Investments required for this alternative are a biogas facility and a gas engine with a generator. The

investment cost for the chosen CHP plant is about 2’000’000 €. Maintenance cost is estimated to be

12.5 €/h (110’000 €/a).


The biogas is in no need of being upgraded, though purification from hydrogen sulphide will

decrease the corrosion and therefore increase the lifetime of the engine. The chosen engine is the JMS

616 GS-B.L which has an electrical output of 2.2 MW an electrical efficiency of 41.8 % and a thermal

efficiency of 42.9 %. This makes a total efficiency of 84.8 %.

7.7 CHP plant with Turbine

In accordance with gas engines, turbines are as well a possible solution to use biogas for combined

heat and power production.




Rasmus Bäckman


A Biogas facility is needed for gas production. The investment cost for a CHP driven by a turbine

is 1’900’000 € (11)

. No specific value for annual maintenance was found, so the cost was set to 0 € to

not overbalance the solution.


The efficiency is estimated in accordance with data from the different sources: Capstone Turbine

Corporation (34)

, Siemens Gas Turbines (35)

and Deublein and Steinhauser (11)

. The electrical efficiency

is estimated to 30 % and the thermal efficiency to 50 %.


The technique that has been chosen for the alternative of upgrading biogas is provided by

Malmberg Water. This technique uses water as absorption liquid. The name of the product is

COMPACT (36)

.

7.8.1 Required investments (37)

A facility sized by the potential biogas production of Chişinȃu WWTP should be able to handle

about 800-900Nm³/h biogas. The investments cost would be about 14’000’000 SEK (1’500’000 €) (37)

.

A facility will be operating for at least 15 to 25 years with proper maintenance (37)

. The operational cost

for upgrading is about 0.27 kWh/Nm ³ electricity

(37), which result in an annual cost of 1’890 MWh/a.


Upgraded biogas could be a renewable energy source as a complement to natural gas. A challenge

is to produce gas that should compete with natural gas. This is hard because of the low price of natural

gas. Another question is how much cheaper biogas has to be if Moldovan Gaz, who has monopoly of

the gas grid, should pay for the biogas that is delivered to them.

7.9 Fuel cell


The investment cost of fuel cells is the major concern about this technique, e.g. the company

Acumentrics are selling a small fuel cell power plant for demonstration (38)

. The price of such a plant is

175’000 USD (137’884 €) for 5 kW (38)

.


Fuel cells have a high efficiency, are very quiet and have no moving parts. But despite the benefits

is this not a solution that will be investigated further in this report. The reason for this is the investment

cost, that the technology is too young and also due to that biogas has to be upgraded to be able to use

as fuel in a fuel cell.

7.10 Sell biogas to a waste incineration plant

This alternative gives APA Canal the opportunity to just focus on the waste water treatment and

sludge management, by letting another company take care of the energy production.


No extra investments have to be done for APA Canal more than an anaerobic digester.




Rasmus Bäckman

7.10.2 Benefits and Comments

There are several benefits of this; no extra investments have to be done to use the biogas, biogas

can be used in the waste incineration plant as supporting fuel and impurities in the biogas are of less

importance because of a modern emission control system at the waste incineration plant. This solution

demands that a waste incineration plant is built. It is hard to know how the market price would be set

for the biogas. The price must be lower than for natural gas but because of the low price of natural gas

it is hard to say if this price will cover the investment cost of an anaerobic digester.

7.11 Incinerate sludge

For this solution it is assumed that the Auto Sanitation Department will build a waste incineration

plant where sludge can be accepted as co-fuel (chapter 2.4).


A solution would be to incinerate the sludge in a major incineration plant where the sludge is just a

smaller part of the total burning material. If a power plant like this would be build next to the WWTP

it would be possible to pay a fee to the company in charge of the plant to get rid of the sludge. A new-

built major plant would have safe and modern tools for emission reduction. To pay a fee for sludge

incineration has to be compared with the money saved by not having to build any new facilities.


This is a very easy way to get rid of sludge and make it possible for APA Canal to just focus on

their main subject which is water treatment. There would be almost no energy gain out of this solution

and there are also some problems with sludge incineration. First, it is not sure how long time it will

take before the incineration plant is completed. A second problem is what such fee would be to find a

price that both companies agree about. A third thing to discuss could be if it is environmental friendly

to just transfer sludge into the atmosphere without recovering any energy.

8 Calculated potential amount of biogas

8.1 Method

To get as accurate value as possible, the biogas potential was calculated by using several sources.

A reasonable value will be found in the interval between these values, because the sources differs in

the value for biogas content (B) in sludge.

Svenskt Vatten AB (13)

says that the biogas content is BTS = 0.33 Nm3/kgTS(primary and secondary sludge).

The same author claims that energy content in biogas is 6 kWh/Nm3.

Deublein and Steinhauser (11)

say that sewage sludge from households contains BVS =0.2-0.75

Nm3/kgVS and sewage sludge from industries contains BVS = 0.30 Nm

3/kgVS. Energy content in biogas

is 6-6.5 kWh/m3.

Values from Bromma wastewater treatment plant according to Bogren, C. (39)

are that for primary

and secondary sludge the value is BVS = 0.51 Nm3/kgVS.

At the moment the flow of primary and secondary sludge is not measured at Chişinȃu wastewater

treatment plant, so two different values are given. These numbers are calculated but they are not exact.

The first is 1100 m3/day

(3)= 401'500 m

3/a. The second value is 293'605 m

3/a

(3). Because of the

problems with these numbers a value of 350000 m3/a was estimated. The TS in the sludge was 5.9 %

in 2008 and 5.5 % in 2009. The organic matter of the TS is 72.6 %. 19 % of the sludge comes from

industries, 75 % comes from households and the remaining 6 % consists of e.g. rainwater. The density

of the dry substance is estimated to ρ= 1000 kg/m3

TS.




Rasmus Bäckman

8.2 Results

From the values above the interval of dry substance (TS) and volatile solids (VS) per year was

calculated:

TSin,min = 19'250 m3/a

TSin,max = 20'650 m3/a

VSin,min = 13'976 m3/a

VSin,max = 14'992 m3/a

Potential biogas extraction according to Avloppsteknik 3 (13)

:

V=TS*ρ*BTS

Vmin = 6'352'500 Nm3

Vmax = 6'814'500 Nm3

Minimum energy content = 38'115 MWh

Maximum energy content = 40'887 MWh

Potential biogas extraction according to Biogas from waste to energy (11)

:

V=VS*ρ*BVS

Vmin = 3'060'635 Nm3

Vmax = 9'962'118 Nm3



Potential biogas extraction according to Bromma wastewater treatment plant (39)

:

V=VS*ρ*BVS

Vmin = 7'127'505 Nm3

Vmax = 7'645'869 Nm3



An average value of the calculations above is 42’000 MWh which will be used as the methane

potential in further calculations in this report.




Rasmus Bäckman

9 Results of economical calculations

9.1 Heat boiler

Electrical energy recovery [MWh/a]: -4’000 = -4’000

Thermal energy recovery [MWh/a]: 42’000*1-11’000 = 31’000

Gas energy recovery [MWh/a]: - 0

Total energy recovery [MWh/a]: -4’000+31’000 = 27’000

Net accounting Electrical energy [€/a]: -4’000*1’330*0.059451 = -316’278

Net accounting Thermal energy [€/a]: 31’000*602*0.059451 = 1’109’471

Net accounting Gas energy [€/a]: - 0

Net accounting per year [€/a]: -316’278+1’109’471 = 793’193

Investment cost [€]: 5.7*106

= 5’700’000

Maintenance cost [€/a]: - 0

Profit [€/a]: 793’193

Payback period with 5% interest [a]: -ln(1-(5700000/793193)*0.05)/ln(1+0.05) = 9.1

Payback period with 18% interest [a]: -ln(1-(5700000/793193)*0.18)/ln(1+0.18) = No payback

9.2 CPH plant from ELTECO (present engines)

Electrical energy recovery [MWh/a]: 42’000*0.93*0.384-4’000 = 10’999

Thermal energy recovery [MWh/a]: 42’000*0.93*0.54-11’000 = 10’092


Total energy recovery [MWh/a]: 10’999+10’092 = 21’091

Net accounting Electrical energy [€/a]: 10’999*1’330*0.059451 = 869’689

Net accounting Thermal energy [€/a]: 10’092*602*0.059451 = 361’201


Net accounting per year [€/a]: 869’689+361’201 = 1’230’890

Investment cost [€]: 5.7*106+20*10

6*0.059451 = 6’889’016

Maintenance cost [€/a]: 100’000

Profit [€/a]: 1’230’890-100’000 = 1’130’890



9.3 CHP plant from MWM

Electrical energy recovery [MWh/a]: 42’000*0.42-4’000 = 13’640

Thermal energy recovery [MWh/a]: 42’000*0.438-11’000 = 7’396



Net accounting Electrical energy [€/a]: 13’640*1’330*0.059451 = 1’078’509



Net accounting per year [€/a]: 1’078’509+264’698 = 1’343’207


5 = 6’600’000


Profit [€/a]: 1’343’207-137’000 = 1’206’207






Rasmus Bäckman

9.4 CHP plant from GE Jenbacher

Electrical energy recovery [MWh/a]: 42’000*0.418-4’000 = 13’556

Thermal energy recovery [MWh/a]: 42’000*0.429-11’000 = 7’018



Net accounting Electrical energy [€/a]: 13’556*1’330*0.059451 = 1’071’867



Net accounting per year [€/a]: 1’071’867+251’170 = 1’323’037


6 = 7’704’110


Profit [€/a]: 1’323’037-110’000 = 1’213’037



9.5 CHP plant with Turbine

Electrical energy recovery [MWh]: 42’000*0.30-4’000 = 8’600

Thermal energy recovery [MWh]: 42’000*0.50-11’000 = 10’000

Gas energy recovery [MWh]: - 0

Total energy recovery [MWh]: 10’000+8’600 = 18’600

Net accounting Electrical energy [€]: 8’600*1’330*0.059451 = 679’998

Net accounting Thermal energy [€]: 10’000*602*0.059451 = 357’894

Net accounting Gas energy [€]: - 0

Net accounting per year [€]: 679’998+357’894 = 1’037’892

Investment cost [€]: 5.7*106+1 .36*10

6 = 7’059’954

Maintenance cost [€]: - 0

Profit [€]: 1’037’892 = 1’037’892




Electrical energy recovery [MWh/a]: -0.045*42’000-4’000 = -5’890

Thermal energy recovery [MWh/a]: 0.045*42’000*0. 9-11’000 = -9’299

Gas energy recovery [MWh/a]: 42’000

Total energy recovery [MWh/a]: -5’890-9’299+42’000 = 26’811


Net accounting Thermal energy [€/a]: -9’299*602*0.059451 = -332’805

Net accounting Gas energy [€/a]: 42’000*311*0.059451 = 775’548

Net accounting per year [€/a]: -465’720-332’805+775’548 = -22’978


6*0.104019 = 7’156’266


Profit [€/a]: -22’978

Payback period with 5% interest [a]: -ln(1-(7156266/-22978)*0.05)/ln(1+0.05) = No payback

Payback period with 18% interest [a]: -ln(1-(7156266/-22978)*0.18)/ln(1+0.18) = No payback




Rasmus Bäckman

9.7 Sell biogas to a waste incineration plant

Electrical energy recovery [MWh/a]: -4’000

Thermal energy recovery [MWh/a]: -11’000

Gas energy recovery [MWh/a]: 42’000

Total energy recovery [MWh/a]: -4’000-11’000+42’000 = 27’000


Net accounting Thermal energy [€/a]: -11’000*602*0.059451 = -393’683

Net accounting Gas energy [€/a]: 42’000*311*0.059451 = 775’548

Net accounting per year [€/a]: -316’278-393’683+775’548 = 65’586

Investment cost [€]: 5.7*106 = 5’700’000


Profit [€/a]: 65’586



9.8 Incinerate sludge

There will be a cost for incineration of sludge, but this cost can be reduced by the present cost for

dewatering and sludge disposal.

10 Conclusions A major problem with some of the calculations above is that it is not likely to be able to sell heat all

over the year to full price. Therefore the income of heat is reduced to half the amount in the table in

Appendix II (compare with the table in Appendix I). By reducing this income it is possible to see that

the solution with the heat boiler would be much more affected than the solutions that produce

electricity as well. This reasoning shows that a biogas facility that produces only heat is not a

preferable solution. According to the calculations the CHP plant from MWM has the shortest payback

period.

The investment cost for a new CHP plant can vary from 900’000 € to 2’000’000 €. The given cost

of 1’190’000€ for the present engines seems rather high. It is therefore preferable to check the market

for possibly cheaper alternatives, though it is beneficial that they are already installed. It must be taken

in consideration that the existing engines are used for two years and have been standing still for several

years which makes it necessary to investigate their mechanical condition.

The results from the calculations show the importance of the interest rate in major investments. The

payback calculations show that the difference between 5 % and 18 % in interest rate often is crucial for

a profitable investment. As the results show, the payback periods are rather long and therefore increase

the risk of the investment. It is essential to see that it is more advantageous with the investments than

just energy recovery. One possible solution may be to get the project co-financed with an international

actor, e.g. the Swedish International Development Cooperation Agency (Sida) or the European

Investment Bank (EIB). This could be a way to avoid the high interest rates in Moldova.

The gas prices are very low in Moldova. This makes upgrading of biogas not profitable. The price

for electricity is four times higher than the price for gas. With this fact and the given energy needs of

electricity and heat, the gas price has to increase by 176 % to give an annual profit comparable to a

CHP plant with a gas engine. The gas price would in that scenario be 858 MDL/MWh.

Some of the solutions demand a waste incineration plant. There is a question whether this facility

will become reality or if it just will stop in the planning phase. Before these solutions can be discussed

it is essential to await a final decision if the waste incineration plant will be built in a close future.




Rasmus Bäckman

11 Discussion

11.1 Profits of an investment that are hard to measure in money

When an investment is discussed it is difficult to know how some values should be measured. The

effect of these values may not be seen in economical evaluations of the feasibility of an investment and

it is therefore important to decide how these values should be treated.

11.1.1 Reduction of odour

It would be favourable to stabilise the sludge, in order to decrease the odour. The present solution

of dewatering sludge inside of Geotubes is just a way of confining the odour. Because of the location

of the WWTP it is important to protect the surrounding city from unpleasant odour. This can also be an

important issue for APA Canal in their struggle to have good relation with the population of Chişinȃu.

11.1.2 Possibility of grease management

If APA Canal decides to invest in an anaerobic digester, there will be new possibilities to manage

grease. Because grease is easily degradable it is very beneficial for the anaerobic digester process.

Grease can therefore be fed directly into the anaerobic digester. Installing grease traps at restaurants

and industries will not increase the biogas production in the anaerobic digester because these substrates

are already included in the sludge of the WWTP, but there is several other advantages that will save

money. By separating grease from the regular wastewater a lot of problems in the sewer pipe network

are prevented. Another benefit is that the organic load of the WWTP would be decreased. This report

has not calculated the economical value of this but it is essential to investigate this before taking a

decision about an economical sustainable solution for sludge management.

11.1.3 Reduction in greenhouse emissions (19)

A benefit with stabilisation of sludge is that methane is not released into the atmosphere from

uncontrolled anaerobic digestion. This uncontrolled anaerobic digestion occurs in sludge, stored before

disposal. Methane has 20 times greater greenhouse effect than carbon dioxide. This means that if it is

possible to avoid uncontrolled methane production there will be positive results for the environment.

The energy from biogas will also replace energy that comes from fossil fuels and will therefore have a

positive impact on the environment as well.

11.2 Energy prices

A major problem about the payback calculations is that they are based on current energy prices.

This is a problem because the payback periods are long, up to twelve years, and it is difficult to foresee

the future energy prices. Even small changes in any of the prices would affect the payback period and

change which alternative that is the most profitable. An increase of the electricity price will, for

example, gain the electricity producing solutions. The fact that the conditions can change rapidly

benefits the alternative which focuses on covering internal energy needs and not are depending on

external variables, this because it is easier to prepare for changes within the organisation.

12 Future work In this project there are several questions that would be of interest to investigate but have not been

within the boundaries of the project. These questions are listed here as future work.

Investigate the content of chemicals and especially heavy metals in the sludge. For what kind of

agriculture is it possible to use the sludge as a fertiliser?

How much energy is it possible to extract by a heat exchanger form the cleaned water that has

passed through the WWTP?




Rasmus Bäckman

Are there other substrates in the region that could be co-digested in the anaerobic digester together

with sludge?

What is the best way of dewater stabilized sludge?

Should there be any treatment of stabilized sludge? (E.g. composting, disinfection, etc.)

Would it be possible to extract energy from composting of stabilized sludge?

13 Acknowledgments We would like to thank all people that have made this project possible especially APA Canal who

has been very supportive, mainly through Natalia Vavelschi and Rusnac Arcadie. Ronny Arnberg at

Borlänge Energi has been inspiring and his contacts have been a key in the process of forming the

project. We also want to thank our supervisor at the University of Skövde, Tomas Walander, and

Ångpanneföreningen's Foundation for Research and Development, ÅForsk.

14 References 1. Nationalencyklopedin. Chisinau. [Online] 12 May 2010. http://www.ne.se/lang/chisinau.

2. Visnevschi, Alexandru. Vice director of WWTP, APA Canal Chisinau. 1 March 2010.

3. Vavelschi, Natalia. APA Canal Chisinau.

4. Virlan, Veaceslav. Chief of Mecanical energetic department, APA Canal Chisinau. 20 April

2010.

5. Rusnac, Arcadie. Chief of Department of quality ensuring, control and regulation, APA Canal

Chisinau.

6. Sergheienco, Victor. Vice Director, Auto Sanitation Department. 24 March 2010.

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http://www.xe.com/ucc/convert.cgi?Amount=1&From=SEK&To=EUR&image.x=40&image.

y=4&image=Submit.

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Introduction. Weinheim : WILEY-VCH Verlag GmbH & Co. KGaA, 2008. ISBN 978-3-527-

31841-4.

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Inc., 2003. ISBN 0-471-20693-8.

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16. Energigas Sverige. Biogasportalen. [Online] 18 May 2010.

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sISverigeOchVarlden/GodaExempel/Biogasanlaggningen%20i%20Goteborg.ashx.

17. Fredriksson, Ola. Gryaab. 11 March 2010.

18. Jacobsson, Emma. Göteborgs Energi. 19 March 2010.

19. Swedish Gas Association. Biogas from manure and waste products - Swedish case studies.

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21. Nationalencyklopedin. Gaspanna. [Online] 5 April 2010.

http://www.ne.se/lang/v%C3%A4rmepanna?i_h_word=gaspanna.




Rasmus Bäckman

22. Condensing Boiler. [Online] 5 April 2010. http://www.condensingboiler.org.uk/.

23. JTI - Swedish Institute of Agricultural and Environmental Engineering. Bioenergipotalen,

Gasturbin. [Online] 13 May 2010. http://www.bioenergiportalen.se/?p=1802&m=1215.

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http://www.vattenfall.se/www/vf_se/vf_se/518304omxva/526164energ/526554gas/526614sxx

pr/index.jsp.

25. JTI - Swedish Institute of Agricultural and Environmental Engineering. Bioenergiportalen,

Gasmotor. [Online] http://www.bioenergiportalen.se/?p=1803&m=1214 .

26. Nationalencyklopedin. Bränslecell. [Online] 13 May 2010.

http://www.ne.se/lang/br%C3%A4nslecell.

27. JTI - Swedish Institute of Agricultural and Environmental Engineering. Bioenergiportalen,

Bränslecell. [Online] 13 May 2010. http://www.bioenergiportalen.se/?p=1814&m=1218.

28. Bjurman, Mathias. Head of the section of power, heat and water, Borlänge energi. 9 April

2010.

29. Svensk Fjärrvärme. [Online] 6 April 2010.

http://www.svenskfjarrvarme.se/index.php3?use=publisher&id=1228&lang=1.

30. Andesson, G. Kalkyler som beslutsunderlag. Lund : Studentlitteratur, 2008. ISBN 978-91-44-

05024-9.

31. Yard, S. Kalkyler för investeringar och verksamhete. Lund : Studentlitteratur, 2001. ISBN 91-

44-01057-5.

32. Müller, Jens. Pro2 Anlagentechnik GmbH. 29 April 2010.

33. Grome, Thomas. Sales/Project Department, Jenbacher Gas Engines, GE Energy. 3 May 2010.

34. Capstone Turbine Corporation. [Online] 26 May 2010.

http://www.capstoneturbine.com/prodsol/solutions/.

35. Siemens gas turbines. [Online] 26 May 2010. http://www.energy.siemens.com/hq/en/power-

generation/gas-turbines/sgt-100.htm.

36. Malmberg Water. [Online] 12 May 2010.

http://www.malmberg.se/module/file/file.asp?XModuleId=4133&FileId=12393.

37. Malmberg, Erik. Export Biogas, Malmberg Water AB. 19 April 2010.

38. Acumentrics. [Online] 12 May 2010. http://www.acumentrics.com/tools-faq-fuel-cell-faq.htm.

39. Bogren, C. Mätning av metanpotentialen hos slam på Henriksdal och Bromma, examensarbete

2007.



Daniel Graan

Rasmus Bäckman

Appendix I – Economical calculations

Solu

tion

Bio

gas-

Hea

t bo

iler

CPH

pla

nt f

rom

ELTE

CO (

pres

ent

engi

nes)

CHP

plan

t fr

om

MW

M

CHP

plan

t fr

om G

E

Jenb

ache

r

CHP

plan

t w

ith

Turb

ine

Upg

radi

ng o

f bi

ogas

Sell

biog

as t

o a

wa

ste

inci

nera

tion

plan

t

Ene

rgy

reco

very

Ele

ctri

city

[M

Wh

/a]

4 00

0 -

10 9

99

13 6

40

13

556

8

600

5 89

0 -

4

000

-

Ene

rgy

reco

very

He

at

[MW

h/a

]31

000

10 0

92

7 39

6

7 01

8

10 0

00

9

299

-

11 0

00

-

Ene

rgy

reco

very

Ga

s [M

Wh

/a]

-

-

-

-

-

42

000

42

000

Tota

l Ene

rgy

reco

very

[M

Wh/

a]27

000

21 0

91

21 0

36

20

574

18

600

26 8

11

27 0

00

An

nu

al

ne

t in

com

e f

rom

Ele

ctro

city

*€/

a+

316

278

-

86

9 68

9

1 07

8 50

9

1 07

1 86

7

67

9 99

8

46

5 72

0 -

31

6 27

8 -

An

nu

al

ne

t in

com

e f

rom

He

at

*€/a

+1

109

471

361

201

26

4 69

8

25

1 17

0

357

894

332

805

-

393

683

-

An

nu

al

ne

t in

com

e f

rom

Ga

s *€

/a+

-

-

-

-

-

77

5 54

8

77

5 54

8

Tota

l ann

ual n

et in

com

e [€

/a]

793

193

1

230

890

1 34

3 20

7

1 32

3 03

7

1

037

892

22

978

-

65 5

86

Inve

stm

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t co

st *

€+5

700

000

6 88

9 01

6

6

600

000

7

704

110

7 05

9 95

4

7 15

6 26

6

5 70

0 00

0

Ma

inte

na

nce

co

st *

€/a

+10

0 00

0

137

000

110

000

Prof

it [

€/a]

793

193

1

130

890

1 20

6 20

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1 21

3 03

7

1

037

892

22

978

-

65 5

86

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rio

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rio

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7,8

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nt

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use

r)

Ene

rgyc

on

ten

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i p

ote

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al

Bio

gas

42 0

00

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Wh

/a

Ele

ctri

city

co

nsu

mp

tio

n f

or

bio

gas

pla

nt

4 00

0

M

Wh

/a (

Gry

aa

b)

He

at

con

sum

pti

on

fo

r b

ioga

s p

lan

t11

000

MW

h/a

(G

rya

ab

)

1 SE

K=0,

1040

19

€

1 M

DL=

0,05

9451

€

1 U

SD=

0,78

7910

€

Pric

e f

or

ele

ctri

city

1 33

0

M

DL/

MW

h

Pric

e f

or

he

at

602

M

DL/

MW

h

Pric

e f

or

gas

311

M

DL/

MW

h



Daniel Graan

Rasmus Bäckman

Appendix II - Economical calculations (Annual net income from heat reduced by half)

Solu

tion

Bio

gas-

Hea

t bo

iler

CPH

pla

nt f

rom

ELTE

CO (

pres

ent

engi

nes)

CHP

plan

t fr

om

MW

M

CHP

plan

t fr

om G

E

Jenb

ache

r

CHP

plan

t w

ith

Turb

ine

Upg

radi

ng o

f bi

ogas

Sell

biog

as t

o a

wa

ste

inci

nera

tion

plan

t

Ene

rgy

reco

very

Ele

ctri

city

[M

Wh

/a]

4 00

0 -

10 9

99

13 6

40

13

556

8

600

5 89

0 -

4

000

-

Ene

rgy

reco

very

He

at

[MW

h/a

]31

000

10 0

92

7 39

6

7 01

8

10 0

00

9

299

-

11 0

00

-

Ene

rgy

reco

very

Ga

s [M

Wh

/a]

-

-

-

-

-

42

000

42

000

Tota

l Ene

rgy

reco

very

[M

Wh/

a]27

000

21 0

91

21 0

36

20

574

18

600

26 8

11

27 0

00

An

nu

al

ne

t in

com

e f

rom

Ele

ctro

city

*€/

a+

316

278

-

86

9 68

9

1 07

8 50

9

1 07

1 86

7

67

9 99

8

46

5 72

0 -

31

6 27

8 -

An

nu

al

ne

t in

com

e f

rom

He

at

*€/a

+55

4 73

5

180

600

13

2 34

9

12

5 58

5

178

947

166

403

-

196

842

-

An

nu

al

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com

e f

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Ga

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5 54

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Tota

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ual n

et in

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/a]

238

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1

050

290

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0 85

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2 42

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Inve

stm

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€+5

700

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6 88

9 01

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600

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7

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7 05

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5 70

0 00

0

Ma

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€/a

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0 00

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137

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Prof

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€/a]

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ha

use

r)

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rgyc

on

ten

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ote

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Bio

gas

42 0

00

M

Wh

/a

Ele

ctri

city

co

nsu

mp

tio

n f

or

bio

gas

pla

nt

4 00

0

M

Wh

/a (

Gry

aa

b)

He

at

con

sum

pti

on

fo

r b

ioga

s p

lan

t11

000

MW

h/a

(G

rya

ab

)

1 SE

K=0,

1040

19

€

1 M

DL=

0,05

9451

€

1 U

SD=

0,78

7910

€

Pric

e f

or

ele

ctri

city

1 33

0

M

DL/

MW

h

Pric

e f

or

he

at

602

M

DL/

MW

h

Pric

e f

or

gas

311

M

DL/

MW

h

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