alternatives to spray irrigation of starch waste based distillery effluent

Journal of Food Engineering 60 (2003) 367–374

www.elsevier.com/locate/jfoodeng

Alternatives to spray irrigation of starch waste based distillery effluent

Minh H. Nguyen *

Centre for Advanced Food Research, University of Western Sydney, Penrith South, NSW 1797, Australia

Received 21 September 2002; accepted 5 February 2003

Abstract

The distillery effluent from a starch waste based ethanol plant has been disposed of by spray irrigation for some time. The al-

ternative methods of disposal are reviewed and their then costs compared for a 50 Ml/year plant. Anaerobic digestion can reduce

pollution load significantly but it requires more investment and increased annual cost. Partial solids recovery by ultrafiltration (UF)

does not reduce the pollution as much, but results in a byproduct for sale and a payback period of under six years for the extra

investment. Further solids recovery by UF and nanofiltration requires a much longer payback period for the extra investment.

Further research is required for more economical solids recovery and better value added byproducts.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Distillery effluent; Starch waste; Solids recovery; Ultrafiltration; Nanofiltration

1. Introduction

Pollution control of existing and proposed ethanol

plants is a major concern in attempting to comply withthe Environmental Protection and Assessment Acts and

Regulations in Australia. From a process engineering

perspective, waste treatment and byproducts recovery

are as important as any other processing steps. In this

paper, alternatives to the spray irrigation operation of

the distillery effluent from an ethanol-from-starch waste

plant are considered. The process for ethanol produc-

tion is outlined. The alternatives are briefly reviewed forwaste treatment for distillery effluents, mainly from

molasses, being anaerobic digestion, incineration and

solids recovery. The process economics of the various

alternatives are then compared for a starch waste based

ethanol plant producing 50 Ml/year.

2. Ethanol-from-wheat starch waste

Starch processing residue is a commercially accepted

feedstock for industrial ethanol. Typically wheat milling

results in 78–80% flour, 19% bran and 1% wheat germ.

The flour is washed with water to produce starch.

* Tel.: +61-2-4570-1343; Fax: +61-2-4570-1579.

E-mail address: [email protected] (M.H. Nguyen).

0260-8774/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0260-8774(03)00059-1

Australia processes 500–600,000 ton of starch per year.

As the effluent still contains some residual starch, it can

be converted to ethanol by fermentation (see Fig. 1).

This starch effluent then can be considered as almostfree feedstock for the ethanol plant.

In Australia, the effluent from a starch plant has been

converted to ethanol at the rate of 18 Ml/year for in-

dustrial uses and a small proportion for blending into

diesel and petrol (Grace, 1994). For higher production

capacity, the fermentables in the starch waste are not

sufficient. The starch waste has to be supplemented with

B grade starch, the less pure grade with small starchgranules.

This ethanol-from-starch waste plant employs starch

waste liquefaction by enzymes at 95 �C and saccharifi-

cation at 55 �C and a multi-vessel continuous fer-

mentation system of about 40 h residence time. Yeasts

are grown in propagation tanks and are added to the

broth at appropriate temperatures. The fermented broth

(called beer) is distilled to recover ethanol and the dis-tillery waste/effluent is also called dunder or stillage. The

yeast goes with the beer for distillation. The distillery

effluent is centrifuged to recover the yeast cell fragments

and subsequently spray irrigated on forage crops. Cen-

trifuges are used to recover the yeast fragments from the

stillage, which is neutralized with lime prior to irriga-

tion. The farm soil was reported to have its pH increased

and it can support the grazing of 500 beef cattle over400 ha (Liquid solar energy, 1993).

mail to: [email protected]

Mixer

Separator

Starch Washer

Gluten dryer

3,240 kg Starch waste

Fermenter Distiller

Cooker Liquefaction

Dryer

Converter Saccharifier

WHEAT FLOUR1,000kg

3,000 kg Clarified Effluent to irrigation

120 kg ETHANOL

150 kg GLUTEN

610 kg STARCH

3000 kg Water

Centrifuge

Yeast cells fibres etc

Fig. 1. Ethanol-from-starch waste process.

368 M.H. Nguyen / Journal of Food Engineering 60 (2003) 367–374

3. Spray irrigation of stillage as fertiliser

The practice of spray irrigation of distillery effluent

has been established for a long time with the ethanol

production from molasses, which is a byproduct fromsugarcane processing.

Molasses stillage, as normally centrifuged to recover

organic solids such as yeast cells, retains up to 1.1%

potassium and 3.1% ash (Chen, 1980). It is trucked as

far as economically possible to spray irrigate over cane

plantations. Costs need to allow for the corrosive nature

of stillage, which requires corrosion resistant equipment

and facilities. The Biostil process, from which stillage ata high solids level is produced, may be more economical

for recycling minerals in this way. The practice varies,

for example in France 2.5–30 m3/ha are sprayed after

harvesting (Lewicki, 1978) and in Brazil, 650–1000

m3/ha are spread about 1–4 weeks prior to planting

(Monteiro, 1975).

The advantages of direct return includes formation of

an initial buffer to the soil with calcium and magnesium,increasing its pH; an increase in crop yield (Monteiro,

1975) and improved soil physical properties, increased

water and salts retention capacity and an increased soilmicroflora population (Chen, 1980).

The disadvantages include problems of strong odour,

insect invasion, eventual increase in soil acidity, salt

leaching and putrescity (Jackman, 1977). Another

problem reported includes the build-up of sulfates.

These sulfates are reduced in the soil to hydrogen sulfide

(bad odour), which is then oxidised into sulfuric acid by

sulfur bacteria in the soil (Sastry & Horanhao, 1964).No similar studies have been reported for the return

of starch waste stillage. However, the same problems

should be expected.

4. Anaerobic digestion of stillage

Anaerobic digestion or fermentation of organic

matter is carried out by a mixed group of bacteria under

an environment without oxygen in basically three steps:

hydrolysis, acid formation then methane formation.The above steps occur simultaneously in the di-

gester. The environmental conditions should be main-

tained to favour both major groups of bacteria: the

acid formers and the methane formers. When the

populations are in balance, the volatile acids produced

are rapidly converted to CH4 and CO2. The acid

formers have an advantage over the methane formers

as they grow more rapidly and are less sensitive toenvironmental conditions. Under conditions inhibitive

to methane formers, acid formers can dominate the

reactor, producing more acids and preventing methane

formers from growing; the digester is ‘‘stuck’’. Pre-

cautions have also to be taken to wash the gases from

acids and bad odours.

For stillages with low animal feed value but high

biochemical oxygen demand (BOD), anaerobic diges-tion can be quite effective (Craigvero, 1982). High rate

processes are well suited to stillages and have significant

benefits when compared to conventional aerobic pro-

cesses. A published example was the 97% chemical oxy-

gen demand (COD) conversion of corn stillage in an

UASB (upflow anaerobic sludge blanket reactor)

(Hitara et al., 1988). Another reported example was the

use of a tower fermenter to treat cane stillage at aloading rate of 5 kg COD/m3 day and getting 94% solu-

ble COD conversion (Barford, 1982).

Since no report has been published on wheat starch

distillery effluent, a pilot scale digester was used to

evaluate the feasibility of such an option over a period

of four months (Nguyen, Grace, & Prince, 1996). The

loadings were 1.5–1.7 kg COD/m3 day and 0.5–0.6 kg

(suspended solids) SS/m3 day. The reduction was con-sidered satisfactory at 89–92% for COD and 90–94% for

suspended solids. Gas production was about 16 l per

litre of effluent.

M.H. Nguyen / Journal of Food Engineering 60 (2003) 367–374 369

5. Incineration

For stillages of high organic contents such as mo-

lasses, wood acid hydrolysis wastes and waste sulfite

liquour, incineration offers a possible positive energy

return and the recovery of the minerals (Charkrabarty,

1964). The heat of combustion of molasses or sugarcane

juice stillage solids is 12,500–15,000 kJ/kg. Combustion

generated adequate heat for a 4 or 5 effect evaporator toconcentrate molasses stillage to about 60 wt.% solids

before incineration (Jackman, 1977).

Molasses or cane juice ash has about 30–40% K2O

and 2–3% P2O5. After dissolving in water and neutrali-

sation with sulfuric acid, about 25–35 kg of high value

potassium fertiliser per 1000 m3 of stillage incinerated

(Charkrabarty, 1964). The product is mainly potassium

sulfate with about 16% potassium chloride and 7% po-tassium carbonate, valued at 120 USD/ton (Paturau,

1982). Refine potassium sulfate can fetch 212 USD/ton

(Huffman, 1978). By absorbing the sulfur dioxide from

the boiler gas, ammonium sulfate can be recovered

(Sastry & Horanhao, 1964).

Special boiler designs are required to handle the high

ash content of stillage: a large chamber is generally

considered desirable. Conventional stoke boilers shouldoperate at lower hearth temperatures to prevent ash

encrustation, in particular with molasses stillage as its

potassium ash fusion temperature is only about 700 �C.An atomised mist boiler had been used commercially to

produce high pressure steam (Chen, 1980). Because of

stricter air pollution guidelines, incineration has to be

considered carefully for any new proposal, which nor-

mally should include an electrostatic precipitator sys-tem. In Australia, a trial of direct combustion was

carried out over some months and problems with ash,

slag and boiler design were reported. It was finally

abandoned in favour of irrigation to recycle nutrients

back to cane fields (Muller, 1995).

Direct combustion requires a suitable system for in-

jection of effluent as fuel and ash handling but results in

extra energy and no effluent discharge. In this case ofdistillery effluent from starch waste however because of

the high level of moisture, it is not further considered.

6. Solids recovery

Molasses stillage is typically centrifuged to recover

yeast cells and fragments. The nutritive value of yeast

from molasses distillery has been reported (Rastogi &

Murti, 1964).

The clarified stillage is then evaporated to 50–65 wt.%

solids, when it becomes quite stable against microbialspoilage and marketed as condensed molasses solubles

or ‘‘vacatone’’ (Bu�Lock, 1979). Feeding tests showed

that it had about 65% of the nutritive feed value of

molasses (Lewicki, 1978). However, because of the highsalt, particularly potassium, content, molasses stillage

used in ruminants is limited to 10% of the diet (less than

2% in pigs) to avoid laxative effects. The actual sale price

is low except when soybean meal is in short supply.

Condensed molasses solubles are viscous, sticky and

hygroscopic hence difficult to handle and may benefit by

an addition of ammonium phosphate or phosphoric

acid. Large evaporators are required because of low heattransfer rate, about 16,000 kJ/m2 h �K at 20 wt.% solids

and 5900 at 60 wt.% for forced convection evaporators

(Charkrabarty, 1964). It has been reported that oil can

be mixed with stillage before evaporation, improving the

heat transfer dramatically. The oil can be pressed from

the dried concentrate slurry in a belt press for reuse.

Cane and beet juices stillage can be treated similarly

to molasses stillage. They can also be mixed with bag-asse fibre or beet pulp residue and dried to produce an

enriched roughage feed. The soluble nutrient content of

corn starch, potato and sulfite waste liquour stillage is

considered not sufficient to justify the cost of evapora-

tion. These stillages are usually centrifuged to recover

the yeast, then treated by the conventional secondary

wastewater treatments, because they were considered

as too dilute for evaporating economically.Total solids (TS) recovery as a clean technology op-

tion, for alcohol-from-starch, has been proposed by

Kim, Kim, and Lee (1996). The stillage was decanted

then ultrafiltered. The retentate was recycled back to the

decanter. The permeate was recycled to the cooking step.

It was found that after recycling for eight times, the ave-

rage production yield (8.8%) was quite similar to the

conventional process (9.0%). The zero discharge systemcaused the fermentation time to increase from 60 h to

over 90 h. It was also reported that the total dissolved

solids of the permeate levelled out at 40 g/l. Further work

to confirm the above results and to evaluate the cost of

longer fermentation time would be most valuable.

Stillage from starch waste distillery is usually even

more dilute, so conventional concentration is uneco-

nomic. Nguyen and Panjeena (1998) experimented withultrafiltration (UF), reverse osmosis (RO) and nanofil-

tration (NF) to treat clarified stillage from starch waste

distillery effluent, which has about 3.3% TS. They found

it was possible to ultrafilter the stillage to recover the

high molecular weight fraction as a retentate solution of

16% solids (suspended solid and soluble protein and fi-

bre). The UF permeate can then be nanofiltered into a

retentate of 10% solids (low molecular weight solubles),and final permeate of about 1% solids (mainly salts).

The use of RO to retain all the solids from the UF

permeate and discharge low BOD water was found to be

very slow. TS recovery can only be considered, for ex-

ample recycling the water to the flour slurry stage, if

the flux rate of RO can be improved to make it more

economical.


The direct recycling of UF permeate needs some in-vestigation as it may cause too much contamination for

the starch cleaning stage.

7. Costing of alternatives

An exercise is now made to cost out the alternatives

for treating the stillage of a hypothetical 50 Ml/year

ethanol-from-starch waste plant.

All costs are reported in Australian dollar (about 0.55

US dollar). The costs are based on published figures

or quotations, with index for 2002.The dilute fermentation product has about 5% v/v

ethanol. After centrifugation, the clarified stillage is

produced at the rate of 1000 Ml/year or 2.9 Ml/day over

345 days a year. It has 3.3% total solids, which include

0.4% of suspended solids, 1.0% of soluble protein and

fibre, and 1.9% of low molecular weight solubles.

8. Spray irrigation costing

This simple disposal technique is by spray irrigation

of the centrifuged effluent on to about 730 ha of adja-

cent land, assumed as being available, similar to theexisting plant mentioned above. The land is laser

levelled to accommodate centre-pivot irrigators. The ir-

rigators are large self propelled, rubber tyre spraying

machines that revolve slowly around a central power

and liquid delivery point. Storage ponds of 70 Ml total

capacity are also constructed to allow for wet weather

periods.

Cost is estimated as 2.0 million dollars based onpublished figure, allowing for 14% increase (How

Manildra handles wastewater, 1995). Allowing for ope-

rating costs, which include chemicals, labour and

power as $140,000. Capital charge is $300,000 taken as

Table 1

Starch waste distillery effluent treatment cost alternatives

Alternative Cost in A$ million

1 2

Irrigation system 2.0 (

Digester 5

Ultrafilter

Evaporator

Spray drier

Nanofilter

Capital required 2.0 7

Annual operating cost 0.14 0

Capital charge at 15% 0.30 1

Gas recovery )Product sales

Annual costs 0.44 1

Pollution load reduction 9

Payback period for extra investment

15% of the capital cost. Annual cost is then totalledas $440,000. Overall cost is shown as alternative 1 in

Table 1.

Provided that the land is successfully managed for

feed crops such as corn, this system may continue op-

eration under Environmental Protection Authority

permit. However, further expansion is limited by the

potential risk of salt and nutrient build-up. Reduction

of the level of solids discharges per irrigated acrein necessary, hence other alternatives should be con-

sidered.

9. Anaerobic digestion costing

The cost of an anaerobic digestion plant can be scaled

up from a published cost of a similarly designed plant at

Allensford, Victoria (Kinhill, Metcalf, & Eddy, 1995).This was for 0.5 Ml/day of dairy wastes, of 16,000 mg/l

BOD, using a digester pond of 8 Ml capacity, having a

depth of 8 m. The capital cost was quoted as $1.5 mil-

lion, with $45,000 p.a. operating cost, achieving 85%

BOD removal and recovering gas worth $63,000 p.a.

(using equivalent gas cost of $5/GJ).

Using a capacity factor of 0.67 and allowing for an

increase of 14%, the capital cost of such a digester forthe stillage is $5.6 million. Operating cost is scaled up

proportionally as $296,000 and gas recovery may be

worth up to $430,000 per year.

The spray irrigation system of $2.0 million is still

required but has little potential problem of soil over-

loading with organic nutrients. Capital charge of 15% of

$7.6 million is $1,140,000. Total annual cost of

$1,576,000 including irrigation, is reduced by the gassale and becomes $1,146,000 shown as $1.15 million

under alternative 2 in Table 1.

Although the pollution load now is reduced by about

90%, according to the data reported in Section 3, the

3 4

2.0) (2.0) (2.0)

.6

0.96 0.96

2.40 3.60

2.75 3.30

1.90

.6 8.11 11.76

.44 1.86 3.04

.14 1.22 1.76

0.43

)4.14 )5.65.15 )1.06 )0.850% 48% 75%

5.8 y 11.5 y


extra capital and the higher operating cost requiredmake this alternative not attractive.

10. Costing for solids recovery by ultrafiltration

TS recovery is expected to create revenues from by-

product sale and zero effluent discharge. However, it

requires a very large capital investment to remove waterfrom the very dilute stillage.

As a compromise, a partial solids recovery is con-

sidered, requiring a smaller capital investment, but still

resulting in a significant byproduct sale and a very weak

effluent going into the spray irrigation system.

For this exercise, only the UF is used to recover high

molecular weight components, which include valuable

proteins, to be sold as an ingredient for animal feed.The mass balance of the process, for a 10-fold con-

centration by UF, based on the experimental data re-

ported byNguyen and Panjeena (1998) is shown in Fig. 2.

The pollution load now is 1.9% TS of 109 t/h, in

comparison with the current 3.3% TS of 121 t/h, a ratio

of 52%. Hence, it can be considered that the percentage

reduction of pollution load is 48%.

The cost estimate is shown in Table 2, and the cal-culations are as follows.

The UF system has to retain 12.1 t/h of high mole-

cular weight solubles at 15.9% solids. Its permeation rate

is 109 t/h of permeate at 1.9% TS. Given an average flux

rate of 18 kg/m2 h, a system of 6000 m2 of membrane is

required, costing A$960,000 including design, manu-

facture, delivery, installation and commissioning (Bolch,

2002).The operating cost includes four operators at

4�A$60,000¼A$240,000 p.a.; membrane replacement

cost at A$130,000 per year; cleaning cost at A$62,000

per year; electricity cost 550 kW� 24 h� 345 days�0.05 A$/kWh¼A$227,700.

Distillery effluent 3.3 % TS, 121 t/h

Ultrafilter Irrigation system, 1.9% TS, 109 t/h

12.1 t/h, 16%TS

8.9 t/h water vapour Evaporator

60%TS, 3.2 t/h

Spray drier

Product 96%TS 2 t/h

1.2 t/h water vapour

Fig. 2. Partial solids recovery by UF.

The retentate enters an evaporator/dryer system ca-pable of concentrating the organic rich stream into a

viscous concentrate of 60% solids, of 8.9 t/h capacity.

An estimate of A$ 2.4 million, is based on a quotation

for a four effect system with the last one being forced

circulation type, including design, manufacture, deli-

very, installation and commissioning (Louey, 2002).

The concentrate is dried in a spray drier of 1.2 t/h

water removal capacity to obtain 2 t/h of animal feedproduct at 96% solids. The system should include low

NOx direct gas burner, cyclone, wet scrubber and

product bag filter. This is estimated for $2.75 million,

including design, manufacture, delivery, installing and

commissioning (Louey, 2002).

Although a pilot scale trial has not been carried

out, the above estimates for evaporator and dryer are

considered reasonable, as molasses stillage has beenevaporated up to 65% solids and liquid milk (a high

protein solution) is commonly concentrated up to 50%

solids.

Drying and evaporating labour cost for 24 h/day, 345

days/year by five operators at $60,000 per year each and

2 half-time electrician/fitters at $80,000, totalling

A$380,000 per year.

Dryer energy requirement yearly includes:

• gas cost at 8 GJ/h� $5/GJ� 24 h� 345 days¼A$331,200

• power, 175 kWh installed

consumed 100 kWh� 0.05$/h� 24� 345¼A$41,400.

Energy and water requirements for evaporator allow

steam economy of 0.25, steam required is 8.9� 0.25¼2.22 t/h.

Steam cost: 2.22 t/h� 24 h� 345 days� 15 A$/t¼A$276,300

Electricity: 70 kW installed

Consumed: 50 kW� 24 h� 345 days� 0.05 $/kWh¼A$20,700

Allow 20 t cooling water/ton vapour andcooling water lost is 2%

Water cost: 2%� 8.9 t/h� 20t/t� 24 h� 345 d� 0.50

A$/t¼A$14,500

Annual operating cost for this alternative, including

the irrigation is $1.22 million (see Table 2).

Capital cost for this alternative, including the irriga-

tion system is $8.11 million.

Capital charge annually: 15% of $A8.11 million¼A$1.22 million

Animal feed at an assumed A$250/t wholesale price

for a protein feed: 2 t/h� 24 h� 345 d� 250 A$/t¼A$

4.14 million.

Table 2

Cost estimates for solids recovery alternatives in A$

By UF only By NF+UF

UF capital cost 960,000 960,000

Evaporator capital cost 2,400,000 3,600,000

Spray drier capital cost 2,750,000 3,300,000

NF capital cost 1,900,000

Irrigation system cost 2,000,000 2,000,000

Total capital cost $8,110,000 11,760,000

Annual capital charge 15% $1,216,000 1,764,000

Irrigation annual operating 140,000 140,000

UF annual labour cost 240,000

Annual UF membrane cost 130,000 130,000

Annual UF electricity cost 227,700 227,700

Annual UF cleaning cost 62,000 62,000

UF+NF annual labour 360,000

Annual NF membrane cost 187,000

Annual NF electricity cost 289,800

Annual NF cleaning cost 72,300

Evaporator/dryer/bagger annual labour cost 380,000 380,000

Annual dryer gas cost 331,200 496,800

Annual dryer power cost 41,400 62,100

Annual evaporator steam cost 276,300 558,900

Annual evaporator power cost 20,700 41,400

Annual evaporator cooling water 14,500 29,800

Annual operating cost 1,863,800 3,037,800


Annual surplus A$ million 4.14) 1.22) 1.86¼A$1.06 million.

Payback period for the extra investment after irriga-

tion is 6.11/1.06¼ 5.8 years, which is shown in Table 1.

This alternative requires large capital investment,

however it provides a revenue stream, which is superior

to the anaerobic digestion alternative.

This payback period is of course sensitive to the priceof animal feed, which at the estimated whole sale price

of 250 A$/T, is considered quite reasonable as it con-

tains valuable protein.

The reduction of pollution load is significant, allow-

ing the factory to continue its operation.

Distillery effluent 3.3 % TS, 121 t/h

Nanofilter

12.1 t/h, 16%TS

18 t/h water vapour Evaporator

60%TS, 5 t/h

Spray drier

Product 97%TS 3.1 t/h

1.9 t/h water vapour

Irrigation

1% TS, 89 t/h

10% TS, 10.9 t/h

Ultrafilter

Fig. 3. Solid recovery by NF.

11. Costing for solids recovery by nanofiltration

After UF, the permeate still contains low molecular

weight organics, essentially pentosans that can be re-covered by NF. From the experimental work reported

by Nguyen and Panjeena (1998), a 10-fold concentration

by NF is feasible, resulting in a retentate of 10% solids

containing mainly pentosans and a permeate of 1%

solids, mainly salts (see Fig. 3).

A costing is done below for the evaporation of

combined UF and NF retentates and spray drying again

into an animal feed.The UF system cost is the same as before.

The NF system converts the 109 t/h stream from the

UF system into a 10.9 t/h retentate stream of 10% solids

and a 98.1 t/h permeate stream of 1% solids. At an ave-

rage flux rate of 14 kg/m2 h, the membrane area required

is 7000 m2. The capital cost is estimated as A$1.9 mil-

lion, based on a quotation for design, manufacture,

delivery, installation and commissioning (Bolch, 2002).

The labour cost for combined UF and NF systems

includes six operators at A$60,000¼A$360,000 per

year, membrane replacement cost A$187,200 per year,power at 700 kW� 24 h� 345 d� 0.05 A$/kWh¼A$289,800, cleaning cost is A$72,300 per year.

The UF and NF retentates are concentrated in an

evaporator of the same design as before, removing 18 t/h


of water vapour, resulting in 5 t/h of concentrate at 60%TS. The capital cost is estimated to be A$3.6 million,

based on the quotation by Louey (2002).

Allow a steam economy of 0.25, the steam cost per

year is: 18 t/h� 0.25� 24� 345� 15 A$/t¼A$558,900.

Evaporator power consumed: 100 kW� 24� 345�A$0.05¼A$41,400.

Evaporator cooling tower water consumed: 2%� 18

t/h� 20 t/t� 24� 345� 0.5 A$/t¼A$29,800.The concentrate enters a spray drier to result in 3.1 t/h

of powder (97% solids) by removing 1.9 t/h of water

vapour. The capital cost is estimated as A$3.3 million,

based on the quotation of Louey (2002).

The dryer energy requirement yearly includes:

• gas cost 12 GJ/h� 24� 345� 5 $/GJ¼A$496,800

• power cost 150 kW� 24� 345� 0.05$/kWh¼A$62,100

The labour cost for evaporator and dryer is the same

as before at $380,000 per year.

The total annual cost for this alternative is about

$3.04 million (Table 2). Allow for the sales of the dry

product at an estimated price of A$220 per ton, the

revenue per year is 3.1 t/h� 24 h/d� 345 days/year�A$220¼A$5,646,960.

After the capital charge, the surplus is about

A$850,000 per year. The payback period for the extra

investment after irrigation is 9.76/0.85¼ 11.5 years.

The residual pollution is 1% (98.1)/3.3% 121¼ 0.246.

The reduction in pollution load is then 1) 0.246¼ 0.754

or about 75%.

This alternative requires too large an investment witha long payback period. More research is required to

improve the flux rate of the NF step. The product price

is also low as the protein is reduced. One approach is to

further ferment the pentosans in the product to produce

value added byproducts.

12. Concluding remarks

(1) Spray irrigation of feed crops is the current and

practical option for treating effluent from ethanol plantusing starch waste. In time, however, with eventual in-

crease in plant capacity, the soil will reach its maximum

nutrient load. Other alternatives will then have to be

considered.

(2) Anaerobic digestion has not appeared to be

competitive in this case, justifying the reluctance of the

processor to consider it further.

(3) Partial solids recovery by UF has the potential ofbeing profitable, depending on the selling price of the

animal feed being 250 A$/t. Although the capital re-

quired is significant, the payback period is not too long.

(4) More solids recovery by NF results in more by-products at 220 A$/t, but the extra investment is too

large and the payback period is considered too long.

(5) This solids recovery by UF alternative should be

seriously considered, while further investigations into

total solids recovery can be carried out. It is reasonable

to wait until the payback period has been achieved from

this investment before proceeding further.

During this time, the use of NF or RO systems, whichtheoretically work economically at low solids level,

should be further researched to increase the permeation

rates and to find profitable uses for the recovered ma-

terials.

(6) Another approach is to look at the beginning of

the whole process. The starch waste itself could be pre-

concentrated by membrane technology, to increase the

initial feed concentration. This should result in corres-pondingly higher ethanol concentration, reducing dis-

tillation energy requirement and smaller membrane

filtration areas later on. This approach deserves con-

sideration in the spirit of improved cleaner production.

Acknowledgements

Thanks are due to Mr. Geoff Grace and Dr. John

Pearce for technical information, Prof. Vigneswaran for

advice and discussions.

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alternatives to spray irrigation of starch waste based distillery effluent

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