second draft report

8/6/2019 Second Draft Report

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Table of Contents1 Introduction..........................................................................................................2

1.1 Opuwo Treatment Plant.................................................................................2

1.2 Historical water quality..................................................................................2

1.3 Plant Procurement and Commissioning.........................................................3

2 Objectives............................................................................................................3

3 Plant Process Description.....................................................................................4

3.1 Raw Water Supply..........................................................................................4

3.2 Pre-Treatment................................................................................................5

3.2.1 Acid dosing..............................................................................................5

3.2.2 Anti-Scalant (Scaling inhibitor)................................................................5

3.2.3 Micro-Filtration.........................................................................................6

3.3 Membrane Process Unit.................................................................................6

3.3.1 Nano-Filtration.........................................................................................7

3.3.2 Reverse Osmosis.....................................................................................7

3.4 Brine Disposal................................................................................................7

3.5 Post-Treatment..............................................................................................7

3.6 Membrane Regeneration...............................................................................7

3.6.1 Membrane flushing (Mechanical Cleaning)..............................................83.6.2 Cleaning in Cleaning (CIP).......................................................................8

4 Process Evaluation...............................................................................................9

4.1 Plant throughput............................................................................................9

4.1.1 Sufficient Supply......................................................................................9

4.1.2 Membrane Permeability.........................................................................11

4.2 Quality of water supplied.............................................................................13

4.3 NF/RO vs. RO only........................................................................................16

5 Plant Efficiency monitoring.................................................................................18

6 Conclusion..........................................................................................................20

7 Recommendations.............................................................................................21

Recommendations

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1 Introduction

1.1 Opuwo Treatment PlantThe Opuwo water treatment plant of Namwater is located in Kunene region, in thenorth-west of Namibia, 720km from Windhoek. The plant make uses of membrane

technology to produce water of a group A classification according to Namibianstandards at a rate of 50m3/h (1000m3/day) to supply the town of Opuwo, which wasestimated in 2009 to have a population of about 12 000. The brine from the plant isdisposed in a near-by river and it is not treated further. The Opuwo treatment plantconsists of a two stage membrane configuration, with the first stage using Nano-Filtration (NF) technology and second stage using Reverse Osmosis (RO)technology. The first stage consists of four pressure vessels and each pressurevessel is having six Nano-Filtration membranes, while stage two consists of fivepressure vessels with six Reverse Osmosis membranes per vessel. Each stage is fedby two high pressure pumps, with one in operation and the other on the stand-by.

1.2 Historical water quality

The town extracts water from an underground aquifer by means of five boreholes,situated next to a small river on the outskirts the town. In previous years, there wasno treatment plant in Opuwo therefore the town was supplied directly with waterfrom boreholes. The Opuwo borehole water is classified as group D water accordingto Namwater standards, meaning that the water was of poor quality, and notsuitable for human consumption. This classification was mainly due to excessivelyhigh concentration of Sulfates (SO42-) as well as high concentration of Calcium andMagnesium, resulting in high total hardness (see Table 1). The situation in Opuwowas said to be threatening a whole range of civil, economic and environmentalrights of residents, as the quality of water was detrimental to the health ofresidents. Apart from health the implications, the water was very hard thereforecausing excessive scaling in the piping system of the town as well as other water

retaining appliances. Scaling in pipes is unfavorable as it causes blockages in pipes,thereafter, excessive leakages and even replacement of pipelines which is costly tothe management of the town.

Determinants Units

Opuwo

Requirements(Class A)

pH

pHUnits 7.81

6 - 9

Turbidity NTU 0.68 1

Conductivity mS/m231.02 150

TDS Calculated mg/l1547.80 -

Na mg/l165.40 100

K mg/l 4.56 200

Ca as CaCO3 mg/l302.98 150

Mg as CaCO3 mg/l 771.7 70

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SO4 mg/l556.81 200

NO3 as N mg/l 4.34 10

F mg/l 0.28 2

Cl mg/l189.60 250

Table 1: Opuwo raw water compared to the requirements of class A

As seen in the Table 1, the poor quality is mainly due to Calcium and Magnesiumhardness, as well as the sulfates as highlighted. The conductivity and Sodium levelsare actually in an acceptable range as they fit in the Group B water classification,which is specified as good quality water.

1.3 Plant Procurement and CommissioningWith the poor quality of borehole water in Opuwo, that has a negative impact onhuman health as well as the economy of the town, the need to build a treatmentplant that can produce Group A water, that is regarded as excellent for humanconsumption, was realized. A membrane treatment plant was then commissioned inSeptember 2008, to produce final water that is of Group A to the town of Opuwo.

The commissioning of this plant was done by Aqua Services and Engineering, wherea treatment plant that exploits the method of Nano-Filtration membrane technologywas commissioned. Initially it was a two stage Nanofiltration membrane processwhich yielded a brine flow of 16m3/h as well as a permeate flow of 50m3/h. The twostage Nano-Filtration process did not produce the desired water quality andtherefore the plant was re-designed. The second stage of Nanofiltration membraneswas replaced with a stage of Reverse Osmosis membranes so as to boost thequality performance of the plant and yield the desired results of Group A water at arate of 50m3/h, and at a recovery of 80%. The plant is a high-tech installation thatrequires skilled operation and maintenance. Initially the operation and maintenanceof the plant was carried out by Aqua Services and Engineering until thecommissioning, where after it was handed over to Namwater. The plant is alsomonitored and operated remotely by management through the SCADA system inWindhoek. The brine with a conductivity of about 10ms/cm is discarded in to thenear-by dry river, without any further treatment.

2 ObjectivesThe objective of this report is to evaluate the current performance of the Opuwoplant with reference to the design specifications. This process evaluation is aimed atreviewing the goal and objectives of the plant, if they are met, and inform the steps

forward as well as the current performance regarding the Opuwo treatment plant.Plant evaluation is termed as the reconciliation, rectification, and interpretation ofplant measurements to develop an adequate understanding of the plant operation.

The results of an evaluation are ultimately used to discriminate among causes fordeterioration of plant performance, operating regions, and possible operatingdecisions. The main purpose of plant evaluation has always been to understandplant operations such that relational models between current operation and designspecifications can be developed. The intended results are better economical

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operations, better control, safer operation and better subsequent design. A stableoperation and understanding of the Opuwo plant is desired.

Measurements taken within the past six months from the plant are the foundationfor the analysis. The strategic data and accompanying logic, upon which theevaluation is based on, are also products of the organized input of multiple

partners, and using the collective knowledge of the plant operators as well astechnical persons, and management who are knowledgeable on the running of theplant. Based on the evaluation, as well as the findings, it is necessary to assess theimpact and progress of the current performance and do necessaryrecommendations to improve the performance. The Conclusion drawn from theprocess analysis may lead to modifications of the plant as well as improved design.

3 Plant Process Description

3.1 Raw Water SupplyRaw water that is supplied to plant is provided by five boreholes that are situatedalongside the river. The raw water is pumped from the boreholes to a 100m 3 rawwater collector reservoir at the treatment plant. The boreholes are operated in anautomatic mode to control the level of the reservoir and keep it above 80%. Theboreholes are pumped at different flow rates therefore they are combined to givethe needed flow rate of 63 - 67 m3/h. The boreholes also differ in water quality;certain boreholes have high conductivity, meaning they hold high concentrations of

Total Dissolved Solids (TDS). Figure 1 below display the conductivity associated witheach borehole and the borehole naming is listed in table 2.

Figure 1: Conductivity on each borehole

Borehole Number Borehole numbering

1 WW22864

2 WW23137

3 WW29045

4 WW29046

5 WW29047

Table 2: Show the naming of boreholes as well as their numbering

The conductivity is high at the last borehole, the further you move alongside theriver, the conductivity increases. The concerned parameters for these boreholes aremainly Sulphates, and total hardness that requires the need of a treatment plant atOpuwo. Figure 2 shows that the borehole five (WW29047) does indeed supply thepoorest quality water as it is also high in total hardness as well as Sulphates.

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Figure 2: Total Hardness and Sulphates vs. Boreholes

There are usually three boreholes in operation that deliver water to the 100m3

collector reservoir, from where it gravitates at a rate of 70m3/h to the plant for

treatment.When the plant is not in operation, there is an emergency by-pass line that directlydraws water from the collector reservoir, to the high lift pumps that will supply theuntreated water to the town. When this mode is functional, the water is not safe asit is the untreated water, with high content of Sulphates.

3.2 Pre-Treatment The purpose of pretreatment is to control and minimize membrane fouling,

membrane degradation and reducing flux decline, thereby increasing the efficiency

and lifespan of the membranes. An accumulation of one or more foreign substance

on the surface of the membrane will result in a loss of rate of flow through the

membrane. This will result in the need for higher operating pressures to maintainthe specified water production, as well as resulting in increased energy

consumption and associated cost of pumping. The conventional pretreatment

process mainly consists of an anti-scalant, acid addition as well as micro-filtration.

3.2.1 Acid dosing

Hydrochloric acid, used for pH correction is continually dosed at a rate of 12.5 l/h, to

lower the pH to 6.60 pH units. The acid used is the commercial 30% HCL that is

necessary to prevent inorganic salts such as manganese, iron e.t.c from

precipitating and causing scaling at the surface of the membranes. The acid is

supplied and delivered by Aqua Services and Engineering. It is dosed from two

5000l tanks via any one of the two dosing, which operates in sequence, when one is

online, the other pump will be on stand-by. Since this is a dangerous chemical,

safety precautions are needed at all times.

3.2.2 Anti-Scalant (Scaling inhibitor)

Anti-scalant used for Opuwo raw water is a Hydrex 4109, which is a liquid

formulation based upon a blend of phosphonates, which are extremely effective in

preventing scale formation. Hydrex 4109 is excellent for sulphate, fluoride and silica

scales, very good for carbonate based and phosphate scales for metal oxides. The

dosing rate is determined by a software program of the suppliers of this chemical

and it is based on the historical water quality of Opuwo boreholes. Currently theanti-scalant is dosed at a rate of 1.2 l/h and has thus far increased from the initial

value of 0.94l/h as indicated in the operation and maintenance manual. According

to the Operating & Maintenance manual, the dosage and type of anti-scalant need

to be verified periodically, at least once in every three months and mostly when the

raw water quality changes and should be adjusted by an engineer until the plant

has reached its dynamic equilibrium. In the case of Opuwo, the main parameter to

monitor in the raw water is the silica, as silica concentrations of above 50ppm is

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known to be very scaling and can negatively affect the membranes. Currently the

silicate concentration is around 48ppm. The anti-scalant is diluted with permeate

water to give a 1:1 ratio concentration.

3.2.3 Micro-Filtration

The raw water from the boreholes does contain a certain concentration of dissolvedsolids as well as suspended solids. Suspended solids may consist of inorganic

particles, colloids and biological debris such as micro-organisms and algae. The

quality of the raw water is also defined in terms of concentrations of total

suspended solids (TSS). Measurement of turbidity in Opuwo is the key test that

generally indicates the quality of the raw water in terms of total suspended solids.

Micro-filtration is a purely physical process in which solid particles are captured on

the surface of the filter, and any particle larger than the pore size of the filter

cannot squeeze through. In other words, micro-filtration process removes only

foulants or materials in a solid phase before they enter the membrane process.

At Opuwo treatment plant there are two sets of micro-filtration pressure vessels inparallel meaning the filters operate at the same time. Each pressure vessel is fitted

with 15 Amazon 6315 series, 40 inch cartridge filters, with a pore size of 5

meaning the cartridges only filter out suspended solids larger than 5. This

cartridge filters are only supposed to be operated when the raw water turbidity is

below 1NTU to prevent frequent blockages of the cartridges that will lead to

frequent stoppage of the plant. The pressure drop across the pressure vessel should

not exceed 10kPa; therefore the pressure drop as well as the flow rate after the

cartridge filters indicated by flow-meter FE100, is an indication of when the filters

are clogged and need cleaning or replacement. The cartridge filters are cleaned by

physically washing the solids particles off with the horse pipe, to prepare them for

re-use.

After micro-filtration, the water is primed to enter the membranes, without any fear

of fouling and scaling that has an adverse effect on membrane performance. After

micro-filtration, the water is stored in a 1000L membrane feed tank prior to being

pumped to the membrane process unit.

3.3 Membrane Process UnitThe membranes are typically housed in cylindrical cartridges, containing spirally

wrapped membrane sheets. The design is two stages Nano-filtration and Reverse

Osmosis membrane system, with a 4-5 array design mode respectively. The overallpermeate flow rate produced by the plant is 50m3/h and a brine rate of 13.5m3/h

according to the design. With reference to the Operating and Maintenance manual,

the desired recovery is 80% for the entire plant. The plant is designed to yield

permeate water quality with a conductivity of less than 800S/cm.

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3.3.1 Nano-Filtration

The filtered feed water from the membrane feed tank is pumped to the first stage

which consist of four pressure vessels. The feed water is pumped to yield a pressure

of 5.8 to 6.1 bar, by two grundfos pumps that operate in sequence (one in

operation, another one in stand-by). This stage consists of 24 Toray SU620 8-inch

membranes, with each vessel consisting of 6 membranes. This stage is designed toproduce permeate at a rate of about 19.5m3/h. the permeate flow rate is adjustable

and is controlled by valve V201 while the feed and brine pressures are controlled by

the needle valve VN201.

3.3.2 Reverse Osmosis

The brine from the NF membrane is blended with the raw water directly from the

membrane feed tank which becomes the feed to the reverse osmosis stage. The RO

membranes are fed by two booster grundfos pumps giving the designed permeate

flow rate of 31.5m3/h and the brine of 13m3/h with the operating pressure between

10.8 to 12 bar. The Reverse Osmosis stage is installed with Toray TMG20-400

membranes as well as Dow Filmtec LE400, 8 inch membranes housed in five

pressure vessels, giving a total of 30 reverse osmosis membranes. The type of the

Opuwo RO membranes is the cross-linked polyamise composite, spiral wound

membranes with the pressure drop of 1.2bar over an element. They are fitted in

Wavecyber PV226RF pressure vessels.

3.4 Brine DisposalBrine is the salt concentrate remaining on the upstream side of the Reverse

Osmosis membrane after the separation process. The brine contains higher

concentrations of salts and other impurities than are found in the feed water and

that is why the brine stream is higher in conductivity and TDS concentrations. TheOpuwo brine is approximated to have an amount of 10mS/cm of conductivity.

Due to the chemical make-up of the brine, it is essential to dispose the brine in a

safe and acceptable method. The brine at Opuwo treatment plant is discharged at a

rate of 13m3/h in to a near-by dry riverbed, which flows in the direction of the

boreholes. The current practice of disposing brine with a significant content of TDS

into a riverbed is not environmentally acceptable as it has appalling consequences.

The surface disposal of brine has the potential of polluting the ground water aquifer

that is used as the feed water to the plant, and this is more likely to result due to

the high salinity of the brine.

3.5 Post-TreatmentPost-treatment consists of stabilizing the water and preparing it for distribution. The

objective of post-treatment is to ensure that the product water is safe to drink. In

the case of Opuwo, currently only disinfection is executed. Disinfection is achieved

through the addition of chlorine gas dissolved in water. This is a normal and

accepted water treatment process and it provide water suitable for immediate

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consumption. Disinfection is employed to prevent any harmful bacteria and/or

viruses to enter the distribution system.

3.6 Membrane RegenerationMembrane fouling has become the most encountered problem in the membrane

treatment process. Depending on the membrane type, feed water quality andprocess conditions, the membrane loses its performance over time due to fouling ofthe membranes. Fouling is basically a process resulting in loss of performance of amembrane due to the deposition of suspended or dissolved solids on the membranesurface, at its pore openings or within its pores. Fouling has a bad effect on thesystems performance as it reduces the permeate water flux and also increases thesalt passage, resulting in decreasing water quality as a result of higher TDS. Foulingnot only reduces the water flux but also changes the retention of the membranes.However it is very unlikely to completely eliminate fouling or totally restore themembrane performance. Controlling fouling is utmost important, one of thetechniques used is membrane regeneration or membrane cleaning. Methodsinvolved in membrane regeneration are membrane flushing (mechanical cleaning)

as well as chemical cleaning (cleaning in place). Membrane regeneration is ofparamount economic and scientific importance and has significant impact onprocess operation. The two methods used in Opuwo for membrane regeneration aredescribed below.

3.6.1 Membrane flushing (Mechanical Cleaning)

Membrane flushing is often described as a mechanical cleaning process wherebythe membrane surface is flushed clean with Reverse Osmosis permeate water.Membranes require occasional flushing to limit biological fouling. Biological foulingcan increase membrane process energy consumption and cause malfunctions to themembranes. Flushing is done with Reverse Osmosis permeate water only, which is

circulated through the nanofiltration membranes for one hour. After Nanofiltration,the CIP tank is than filled again with Reverse Osmosis permeate that is to becirculated for another hour in the Reverse Osmosis membranes. On average, themembrane flushing takes about three to four hours, and this process is done on aweekly basis.

The membranes should not remain with brine inside when not in operation,therefore membrane flushing is recommended when the plant shutdowns, even fora small period.

3.6.2 Cleaning in Cleaning (CIP)

Chemical cleaning is a process whereby impurities or foulants by means of a

chemical agent. Opuwo treatment plant requires cleaning of the membranes at amonthly interval. This is done in two phases, by mixing with the low pH cleaning

agent with about 4000 liters of RO permeate water and repeating the same step

with high pH cleaning agent.

Making up the low pH solution requires filling up the CIP tank with RO permeate

water to about 80% full and add 30% Hydrochloric acid (HCL) solution. The acid is

added to bring the pH to between 2-3 pH units or up to the target of 2.5 pH units.

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The solution is pumped into the Nanofiltration membranes (1st stage) for about an

hour and after an hour of recirculation the tank is then emptied/drained. After the

low pH cleaning, the CIP tank is again filled with RO permeate and a high pH

solution is prepared. The high pH cleaning is done with 75 kg of STPP (Sodium

tripolyphosphate) plus 37.5kg Na-EDTA (Ethylenediaminetetraacetic acid disodium

salt). The solutions pH is adjusted to obtain a target pH of 9 pH units (lowering byadding HCL, and raise by adding Sodium Hydroxide (NaOH)). The solution is then

circulated in the Nanofiltration stage for another hour.

The second stage also uses the same procedure when cleaning take place. The

Reverse osmosis stage also has a high and a low pH cleaning which is done the

same way as that of the first stage (Nanofiltration). The low pH of the Reverse

Osmosis stage requires a target pH of one pH unit, while the high pH requires a

target pH of 12 pH units. The low and high pH cleaning each takes about an hour of

recirculation.

The entire cleaning therefore results in about 16, 000 liters of cleaning effluentbeing generated every month and form part of the waste water being disposed to

the river without any treatment.

4 Process EvaluationCertain restrictions imposed by nature must be taken into account whenanalyzing/evaluating an existing plant. You cannot, for example, specify an input tothe plant of 1000m3 of water and an output of 2000m3 of water or anything else.

The basis for both of these observations is the law of conservation of mass, whichstates that mass, can neither be created nor destroyed. Statements based on the

law of conservation of mass such as total mass input = total mass output areexamples of mass balance or material balance. The analysis of an existing one isnot complete until it is established that the input and output of the entire process orindividual unit balances, satisfy the balance equation. By accounting for materialentering and leaving a system, mass flows can be identified which might have beenunknown, or difficult to measure without this technique. The performance analysiswas performed around the Namwater Nano-filtration/Reverse Osmosis purificationplant in Opuwo, to evaluate and analyze the performance of the plant.

In addition, Total Dissolved Solids (TDS) as the parameter representing soluteconcentration and data from the design manual were used to verify the designanalysis, while data from the experiments and from SCADA correspond to the

analysis of the plant in operation for the past six months.

4.1 Plant throughput Throughput is the amount of output that can be produced by a system or

component in a given period of time. It has a meaning similar to that of capacity,

and the two are often used as synonyms. In Opuwo, the plant was designed to a

capacity of 50m3/h, with a daily target of 1000m3/day. This amount is basically

derived from the consumption of water in the town of Opuwo.

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4.1.1 Sufficient Supply

The final water of Opuwo is pumped to a reservoir in town which then is distributed

to the town by gravity at a rate of about 80m 3/h for 19 hours a day. Therefore the

Opuwo residents consume about 1500m3/day, excluding the Opuwo district hospital

and the Opuwo country hotel. Therefore the demand of water to the town is more

than that what the plant can supply. Since the plant is designed to produce 50m3

/h,on a full day operation without any stoppage, the plant can produce 1200m3/day.

Furthermore the design throughput of 1000m3/day is not always achieved due to

certain delays caused by plant stoppages. There are a number of factors that

contribute to plant stoppages, and reduced runtime of the plant. The graph below

shows the amount of water supplied to the town per month compared to the

specific monthly target.

Figure 3: Monthly production

According to the graph above, during the months of March to the June, the plant did

not reach or even exceed its production target. This is mainly caused by the plant

stoppages as it limits the amount of time the plant is expected to be in operation.

There are a number of factors that affects the run time of the plant, and these

factors are listed in table 3 below.

1. Membrane

Cleaning

According to the plants Operation and Maintenance

manual, the Chemical cleaning should take at least four

hours. One hour NF low pH cleaning, another hour NF

high pH cleaning and the remaining two hours are for

RO low and high pH cleaning respectively.

Currently the situation on the ground is that the

cleaning takes about nine hours. This is due to cleaning

of individual pressure vessels, therefore the process

takes about two hours to do low pH cleaning on one

stage only.

The manual states that the cleaning is automatic, which

is basically not the case, as an operator needs to fill up

the tank manually and empty it manually and fill it up

again by restarting the plant. This entire process

consumes time.

2. Cartridge filters There are certain days when cartridge filters block

several times in a day. When the cartridge filters clog,

they reduce the feed flow to the plant, and therefore

they need to be replaced with cleaned cartridge filters.

Replacing cartridges takes about 30min to 40min.

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During the days when the water is very fouling due to

high turbidity, the replacement of cartridges takes

about two to three hours a day.

3. Boreholes

breakdown

The borehole pumps do experience breakdowns

occasionally, when two or three boreholes that supply a

large amount of water to plant are not in operation, this

affects the supply of water to the collector reservoir,

and causing the level of the reservoir to drop. Since the

reservoir feed the plant by gravity, the level of the tank

has an effect to the plants feed water supply rate. If the

level drops, the raw water supply rate decrease,

resulting in insufficient water supply to the plant.

4. Insufficient

chemicals

This happens when the chemicals specifically

Hydrochloric acid are either ordered late or being

delivered late. This causes the pH to excessivelyincrease up to 7pH units which causes the plant to

automatically shutdown. The plant will not start until the

pH drops to below 7pH units.

5. Insufficient raw

water

When the supply of raw water to the plant is insufficient,

the rate at which the membrane feed tank level drops

increases. As the tank level drops to below 50%, the

plant automatically shutdown.

To prevent the membranes from the hydraulic shock

due to frequent start and stop of the plant, it is

recommended to activate a by-pass to find the solution

to the problem, which is mainly caused by air-locks or

cartridge filter blocking.

6. Low level of the

towns reservoir

When the reservoir in town drops to below 20%, the

plant is mainly stopped and the by-pass is activated.

This is mainly due to the water demand is higher than

the supply.

Table 3: Factors that affect plant's availability

1.1.1 Membrane PermeabilityReverse osmosis and nanofiltration system performance is mainly characterizedby two parameters, permeate flow and permeate quality. Membranepermeability can be described as the rate of flow of water through the semi-permeable membrane and often defined as an amount of waterproduced/permeating per unit area and time. The permeability of a membranemainly depends on the feed pressure as well as the osmotic pressure and it isnegatively affected by concentration polarization and membrane fouling to such

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an extent that the membranes deteriorate and become un-operational. Thepressure needed to produce the required permeate flow for a specificmembrane depends also on the designed permeate flux. The higher thepermeate flow, the higher the feed pressure, therefore pressure and flux aredirectly proportional.

Reverse osmosis and nanofiltration performance is typically evaluated by thechange of flux and product quality over time. The change of product quality forthe Opuwo plant will be discussed in section 4.2 under quality of water supplied.The membrane flux always decline over time. This is due to membrane foulingand therefore less is water passing through the membrane. When flux decline isnoticed, membrane cleaning is performed to restore the flux, but due topermanent fouling of membranes, the flux continue to decline further up to thepoint where the membranes have fully degraded and are no longer in acondition of operating and this is termed as a critical flux. One of the ways oflimiting operation costs is to operate at a constant filtration flux below thecritical flux. However, due to the presence of scalants and foulants in raw

waters, the critical flux is quite low which implies an increase in membrane areaand thus in investment cost. Therefore operation is basically above the criticalflux and once the actual flux of the membrane reaches the critical flux, thesedenote that membranes need to be replaced.

Figure 4: RO and NF Flux over time

Figure 4 illustrate the decline of the RO flux and an increase in NF flux. RO has a

higher rejection and recovery than the Nano-filtration, which is why the RO is more

exposed to scalant and foulants. This is because the overall plant is operated at a

constant flux rate and producing 50m3/h. The decline in RO flux will eventually

causes a decline in an overall flux of the plant; to maintain the constant flux of the

plant, the NF flux need to be increased and the Nanofiltration flux will compensate

the loss of flux from the RO. The graph indicates that as the RO flux decline, the NF

flux increase; therefore there is a constant outflow of water from the plant which is

50m3/h. The green arrow on the graph indicates the date when the RO flux was

equal to the NF flux.

Figure 5: The day when the membranes need replacement

According to the Filmtec membrane specifications and technical manual, the

minimum flux of the RO membranes should be 15% lower than the original flux. The

Opuwo RO stage was designed to produce 30m3/h therefore when the RO permeate

flow reaches 25.5m3/h, the membranes are fouled beyond restoration and need to

be replaced. The graph (figure 5) illustrate that the RO membranes will be

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permanently fouled and be replaced on the 24 th February 2011 as per current flux

loss rate.

1.2 Quality of water suppliedThe Opuwo treatment plant was designed to produce water of Group A quality to

the town of Opuwo at a rate of 50m3

/h. Currently the plant is not performing up tostandard as the water being produced is classified as Gruop B. The salts rejection

has thus far decreased with 13.2% from the previous year (2009).

Year

Totalhardness -Permeate(mg/lCaCO3)

TotalHardness Feed (mg/lCaCO3)

Classification

HardnessPassage(%)

Percentageincrease(%)

2009 240.58 1076.67

A22.35

42.35

2010 362.08 1138.33 B 31.81Table 4: The Change in Total Hardness

Table 3 indicates the enormous increase in total hardness of the permeate water,

furthermore the total hardness (Ca + Mg) passing through the membranes has

increased with 42% over the two years. The classification of the Opuwo final water

as Group B was due to the Total Hardness that is above the limit of 300 ppm

(CaCO3). The data above are the yearly average from the main data base of

chemical analysis. The passage of salts through the membrane depends on the

performance of the membrane. If the performance of the membrane deteriorates,

the salt passage increases and the result is poor water quality.

Figure 6: Increase in Conductivity

The main measurement to indicate the amount of dissolved salts in the water is by

conductivity analysis, which is converted by calculation into Total Dissolved Solids

(TDS). This represents the amount of salts in the water. The conductivity of the

permeate water is increasing, thus the quality of the water produced is decreasing.

Figure 6 indicate that the increase in water conductivity might be due to the

increase in feed water conductivity. This is due to the design inaccuracy as the

maximum raw water design conductivity was listed as 2450S/cm, which in actualfact is very low compared to the current conductivity of the raw water which ranges

from 2800S/cm to 3000S/cm.

The table 4 shows the rejection rate for individual pressure vessels and as

illustrated in the table, the reverse osmosis A and B pressure vessels are performing

poorly compared to the other RO pressure vessels. According to the operators at

the plant, the RO pressure vessels consist of different types of membranes A and B

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pressure vessels consist of Toray TMG20-400, whilst C, D and E Reverse Osmosis

pressure vessels consist of Dow Filmtec LE400 membranes. Thus the Toray

membranes are performing worse with the current raw water quality when

compared to the Dow Filmtec membranes.

Nano-Filtration Vessels Average Reverse Osmosis Vessels Average

A B C D

24.36

A B C D E

94.84Rejection%

18.70

31.18

25.98

21.59

90.95

91.20

97.65

96.56

97.85

Table 5: The average rejection rate for individual pressure vessels

Figure 7 illustrates the increase of salts passage through the membranes. The

amount of salts passing through the membranes increase therefore RO membranes

performance also decreases. Hence the rejection of the membranes has also

decreased and it is therefore not constant. The performance of the RO membranes

has been discussed earlier in that the performance is decreasing due to the

decrease in flux. The plant has a constant flux therefore the solute passage of the

entire plant has to increase because of the NF flux increase, and the NF has a very

low rejection compared to the RO. The increase in salt passage is also evident as

there is an increase in the conductivity of the permeate water.

Furthermore, the cause of the decrease in Opuwo water quality might be due to the

decrease in the RO flux and this was worsened by the decrease in raw water

quality. The increase in the NF flux basically also have an effect on the quality of the

water being produced as the NF has a low rejection therefore the salt passage of

the NF is high. The increase of flow in the NF permeate has thus far increased the

overall salt passage of the plant as a whole causing the quality of the final water tobe worsening.

Figure 7: Graph for Solute passage over the membranes

1.3 NF/RO vs. RO onlyThe Opuwo plant consists of a two stage membrane system whereby the first stage

is the Nano-filtration process and the second stage is the reverse osmosis process.

With this current design, the overall flux of the plant is constant as the permeate

flow from the Nano-filtration is adjustable to maintain the constant final water

output flow. The issue with this current design has an effect on the quality of thewater as the RO membranes begin to clog, the flux of the RO drops and the flux of

the NF has to be adjusted upwards in order to maintain the plant production flow at

50m3/h. Since the NF has poor permeate quality, the increase in the NF permeate

flow will automatically affect the quality of the final water.

There are provisions for modification of the plant, which include a system with

Reverse osmosis only. The design of the RO system has been described below. The

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reasons for modifications are to improve the water quality and maintain Group A

water. To maintain the Group A water quality, the RO system will be designed with a

constant recovery. A system with high permeate flux rate is therefore likely to

experience higher fouling rates and more frequent chemical cleaning, whereas the

system with a low recovery rate will not be economical. Therefore the conservative

approach is to design a system with an optimum flux in order to enjoy a trouble freesystem operation and an increased membrane lifespan.

Considering the feed source and the quality of the feed water, the suitable flow

configuration will be the current system, which is continuous operation where the

operating conditions are kept constant with time. The membranes needed for

brackish water are the current 8inch RO Filmtec LE 400 elements; these

membranes have a surface area of 37.2m2. The current permeate flux is

0.025m3/m2.h and a recovery of 80%.

Using the permeate flow of 50m3/h, the number of RO elements will be:

Dividing the permeate flow rate QP by the design flux JW and the membrane surface

area of LE 400 will give the number of elements needed NE.

NE=QpJW.SE=500.02537.2=54 elements

The number of pressure vessels:

The number of elements NE divided by the number of elements per pressure vessel

NEPV. For larger systems, 6 elements vessels are standard therefore NEPV is 6

NV=NENEPV=546=9 PV

According to membrane manufactures, 80% recovery for the system of

underground feed requires a two stage system. The number of stages defines how

many pressure vessels in series the feed will pass through until it exits the system.

The relation of the number of pressure vessels in subsequent is called the staging

ratio R. the ideal staging of a system is such that each stage operates at the same

fraction of the system recovery. The staging ratio R of a system with n stages and a

system recovery r can be calculated:

R=[11-r]1n

R=[11-0.8]12=2.24

The number of pressure vessels in the first stage NV(1) is calculated with the staging

ratio R from the total number of vessels NV.

NV1=NV1+R-1

NV1=91+2.24-1=6PV

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Therefore according to design, the system will consist of 6 pressure vessels in the

first stage and 3 pressure vessels in the second stage, meaning that for the current

flux and recovery, the system design will be a 6:3 array system. The chosen system

must then be analyzed using the Reverse Osmosis System Analysis (ROSA)

computer program. This program calculates the feed pressure and estimate the

permeate quality of the system as well as operating data of all individual elements.It is then easy to optimize the system by changing the arrangement of the

membranes to yield the desired results.

Due to the pressure drop in the feed/brine channel and the increase of the osmotic

pressure from the feed to the concentrate, the permeate flow rate of the elements

at the concentrate end will be lower than the flow rate of the lead elements. The

goal of the design is to balance the flow rate of elements in different positions. This

is achieved by the following means:

Boosting the feed pressure between stages (preferred for efficient energy

use). Applying a permeate backpressure only to the first stage (low system cost

alternative).

The difference between the NF/RO system and the RO membranes system only,

operating at the same permeate flux and recoveries are:

The RO system only will produce far better water quality then the

combination of NF and RO because RO membranes have a higher rejection

rate than the NF.

The RO membranes only will produce a more constant permeate quality than

the NF/RO system since the RO permeate flux can be variable. The lead elements of the RO membranes only are exposed to better water

quality as they receive the normal feed water, unlike the current system

where the RO receive water with the conductivity of about 350mS/m.

Therefore the replacement frequency will be less when compared to the

current system.

List of Symbols Definition

SE Membrane surface area per element

(m2)

QP System permeate flow (m3/h)

NE Number of elements in system

NV Number of six element pressure

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vessels in system

NV(1) Number of pressure vessels in first

stage of 2-stage system

JW Permeate flux of the system (m3/m2.h)

NEPV Number of elements per pressure

vessels

R Staging Ratio

r System Recovery (expressed as a

fraction) = permeate flow/feed flow

Table 6: Symbols definitions

1 Plant Efficiency monitoringEffective operation and maintenance of a reverse osmosis water treatment plant

often revolves around the ability of administrators and key personnel to make

informed decisions in a timely manner. RO membrane systems have long relied

upon data normalization to simplify raw operational data into meaningful and

accurate trends. The performance of the membrane degrades with time due to

fouling or deposition of materials on the membrane surface. The fouling of

membranes results in a significant cost impact in terms of flux/production losses,

energy efficiency, and maintenance costs. Plant efficiency monitoring has the ability

to diagnose any deviation in membrane performance and the causes as well.

Performance monitoring also helps detects the change in the raw water quality so

that possible adjustments can be made in terms of chemical dosage and operatingparameters.

In order to maintain the optimum efficiency of the membranes, plant performance

characteristics must be monitored and stored, at least occasionally or at the best

continuously. This performance information must include the following parameters

in a form of graphs.

Plants Availability: this graph indicate the running hours of the plant and

how many hours the plant has stopped on a monthly basis. The number of

hours that the plant stopped can either be collected from the SCADA

information or can be manually entered from the log-sheet. Permeability: this graph shows the performance of the membrane

throughout the month. The graph has a format as the graph in figure 5. The

importance of this number is that it measures how fast the water permeates

through the membranes. This graph also indicates the date as to when

membranes replacement need to be prepared for.

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Recovery: recovery is also another variable to be monitored on a monthly

basis. Recovery is the ratio of the permeate flow to the feed water flow and is

given in percentage.

Salt Passage: this graph plots the percent salt passage of the membranes,

a format of this graph is shown in figure 7. The importance of this value

measures the efficiency of the membrane on how fast the salts pass throughthe membranes. This number is also affected by the ionic make up of the

feed water.

Conductivity: this graph shows the conductivity of the feed as well as

permeates, and the format is shown in figure 6 of this document. This graph

will help to detect the increase or decrease in the quality of water produced

by the membranes and the quality of water supplied by the boreholes.

Production: this graph is shown in figure 3, and show the amount of water

produced in a month.

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1 ConclusionIn conclusion, the Opuwo plant has not being performing according to the design

and its purpose of implementation as it is suppose to produce Group A water at a

rate of 1000m3/day and instead it is producing Group B.

1) This is due to the decline of the Reverse Osmosis flux that is mainly caused by

membrane fouling. The RO flux decline has caused the NF flux to increase and

therefore maintaining the same overall plant flux as there is a constant final

water outflow. Maintaining the quantity produced by the plant constant has thus

far a negative impact on the quality of the water.

2) There are three main causes of the RO flux decline, which are:

The method of membrane cleaning currently being utilized, as it is not the

same method as stipulated in the operation and maintenance manual for

the plant.

The other noted factor is the membrane preservation. The Opuwo

membranes are not preserved in a relevant solution during lengthy plant

shutdowns, as the recommended solution basically prevent biological

growth in membranes and biological growth can cause serious harm to

the membranes.

The third factor is the membrane flushing before shutdown. It has been

boldly noted in the operation and maintenance manual that membranesshould not be left standing with brine and therefore membrane flushing

with RO water should be implemented, even if the plant is shutting down

for shorter periods of time.

1) It is further concluded that the cause of the poor performance of the

membranes are due to the change in raw water quality. The manufactures have

designed the membranes with lower than current maximum raw water

conductivity. The information supplied to the designers was incorrect, as the

maximum raw water conductivity was noted as 2450S/cm whereas the raw

water conductivity stands at about 3000S/cm. The raw water quality has a

significant impact on the permeate water quality.2) Due to many prolonged stoppages, the plant has not been achieving its monthly

target. The causes of the plant stoppage are stipulated in table 2 of this

document and further recommendations are stated below. One of the causes of

the plant shutdown is the final water reservoir in town running low, therefore it is

concluded that the water demand of the town of has increased, exceeding the

supply rate.

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8) It has been noted in the operation and maintenance manual the anti-scalant

type and dosage needs to be verified periodically, at least once every three

months. But the type and dosage of the anti-scalant has been constant for quite

a long time even when the raw water quality has changed. Therefore major

adjustments to the operating parameters and chemical dosages, even possibly

changing the type of anti-scalant is required as the raw water quality haschanged. The adjustments should be done until the plant dynamic equilibrium is

reached, as the current operation and dosage is based on the design raw water

quality which has changed over time.

9) The operation and maintenance manual indicated that the cartridge filters

should only be used when the raw water turbidity is below 1 NTU. The raw water

turbidity is not measured, as the only measured turbidity is the feed water

turbidity after the cartridge filters. Therefore the raw water turbidity before

cartridge filters need to be measured for the protection of cartridges.

1

second draft report

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