Fish processing wastewater: Production of internal carbon source for enhanced biological nitrogen removal

8)

Pergamon

Wal.Sci.Tech. Vol.32, No. 9-10, pp. 293-302, 1995. Copyright e 1996 IAWQ Printed in Oreal Britain. All rights reserved. 0273-1223/95 $9'50 + 0 00

PII:S0273-1223(96)OO102-3

FISH PROCESSING WASTEWATER: PRODUCTION OF INTERNAL CARBON SOURCE FOR ENHANCED BIOLOGICAL NITROGEN REMOVAL P. Battistoni and G. Fava Department of Earthand Material Sciences, University of Ancona, Via BreeceBianche,60131 Ancona, Italy

ABSTRACT A plant treating fish processing wastewaters was monitored overoneyear to provethe practical advantages achievable through the internal production of RBCOD. A clear improvement of the effluent quality with lower NOx-N content and the disappearance of fluctuation of nitrateswereobtained. Changingthe equalization management from aerobic to anoxic,it was possibleto utilisebatteredwasteas internalcarbonsource. Plant reliability was confirmed during the entireobservation period. The plant broke down twice electromechanically and it required more than 24 days to restore normal plant conditions. The advantagesobtainedovercome the risksassociated with the newscheme adopted.

KEYWORDS Fish processing, nitrogen biological removal, RBCOD production, wastewaters

INTRODUCTION Fish processing wastewaters have a high content of COD, nutrients, oil and fats when fish evisceration and cooking are undertaken (Mendez et aI.; 1992; Aguiar et aI.; 1988). Industries addressed to the preparation of fish retails work on frozen fish and produce lower strength wastewaters according to high specific water consumption (SC) in defrosting and cleaning, and to the small amount of organic matter released from processed fish (Battistoni et aI.; 1992 a-b). Generally speaking, oil concentration, up to 100 mg I-I, does not add to management difficulties in the treatment plant; however a high nitrogen removal efficiency must be assured since Italian industries are mainly located near the Adriatic seashore. The Adriatic sea is retained a sensitive area to eutrophic phenomena and wastewaters discharged from treatment works will have in 1998 more strict nitrogen and phosphorus standards, according to the EEC 271/91 Directive. Activated sludge process with biological denitrification is a widespread technology satisfying all these requirements; furthermore a fast denitrification rate is reached when enough readily biodegradable COD (RBCOD) is available (Ekama et aI., 1991; Henze, 1991). The activated sludge technology is dominating over biofilm processes in the practical treatment of fish processing wastewaters (Battistoni et al., 1992a, 293

294

P. BATIlSTONl and G. FAVA

1993), even if an adequate discharge organisation is requested to assure plant efficiency when a seafood prefrying production facility exists (Battistoni and Fava, 1994a). For enhanced biological nutrient removal plants (BNR) it has been reported that fermentation of primary (Lotter et al., 1992) or biological sludge (Kristensen et al., 1992) may be utilised as internal carbon source to produce low molecular weight substances . In this wayan increase of phosphorus removal in the anaerobic step or a high COD I N ratio in denitrification were obtained together with the maximum denitrification rate (Ekama et al., 1991). A denitrification improvement through internal production of RBCOD in fish processing wastewaters, utilizing the battered wastes from seafood prefrying as carbon source and the equalisation basin as anoxic reactor, has been reported (Battistoni et al.; I 994b). This enabled increments ofinfluent RBCOD ranging from 17 to 55% when a hydraulic retention time (HRT) of 0.63 d was employed, and to measure a fast denitrification rate of2.5-6 mg N03-N/g MLVSS *h and a lower rate of 0.5 mg N03-N/g MLVSS *h. In this one year research, a BNR plant treating fish processing wastewaters has been followed with the aim of individuating the best operative conditions able to guarantee nitrogen removal. The plant reliability connected with the use of internal carbon source and the risks due to a complex plant management or the break down of electromechanical systems were examined.

EXPERIMENTAL RBCOD measurements were made following two methods: the first is the simplified procedure of Mamais et al., (1993), where non biodegradable COD is measured in the plant effluent; the second is based on anoxic batch tests as described by Ekama et al. (1986). Denitrification curves were obtained with and without nitrate addition at anoxic conditions, for calculating the fastest (KI) and slowest (K2) constant rates. Chemical-physical characteristics of wastewaters were measured as described in Standard Methods (1985). Flowrate and temperature values were collected using Metrosonic data loggers. VF A, ethanol and methanol determination were peformed using a gas-chromatographic method (column.Nukol 15m, temp. 85-125 °C, 30 °C/min, carrier:N2, 5 ml/min); ethanol and methanol ( PoropaKT 50/100 mesh, 6*1/4*2mm deactiglass, Temp 140°C oven, 250 injector, 300 Fid, Carrier N2 30 mVmin). Batch tests on samples supplied from different streams (B,BI,A and C Fig. I) incoming to the wastewater plant, were conducted in a 5 I flask with slow mixing to determine anoxic conditions and in a thermostatic bath (20°C). BNR plant was monitored according to the plan in Table I.

RESULTS AND DISCUSSION

Factory and BNR plant The factory, located in the middle of the Adriatic coast ofItaly, is addressed to the preparation offish retail. The global amount of fish processed,15-20 x 103 tonsly, represents a conspicuous percentage of the Italian production in the sector of frozen fish. However, the seafood production is gradually evolving; the most recent organisation of water consuming production is made up of two lines: line A, cuttle fish, squid, octopus cleaned and frozen on ship or cuttle fish, frozen without any cleaning; line B, prefrying frozen cod. In actual fact while line A employs water for defrosting, cleaning the raw fish, or for cleaning up spills, floors and machines, line B consumes water only for these last service operations. The wastewater sewer follows a separate scheme for sending the discharges (8, Fig.l) with a high oil content to an A.P.I. separator, at the same time A and B] wastes are pumped to an equalisation basin. BNR plant is constituted of an extended aeration process (800 m3) with predenitrification (240m3) (Fig. I). Thickening (13 m3) and dewatering of waste sludge are also operative. The peak flowrate, and daily and weekly variation in production load, are partially controlled with an equalization basin (500 m 3) feeding the

295

Fish processing wastewater

plant with a constant flowrate of 33 m 3/h. Almost continuous feeding is observed. Moreover, the treatment plant is fed for six days against the five days of industrial production. TABLE I Plan ofBNR plant monitoring nO analyses/week

Parameter

2 2 on line on line on line 3

MLSS,MLVSS COD, SCOD,RBCOD Temperature pH influent influent flow NH4,N02,N03,TKN CI,S04,P04 Solid waste sludge

3 daily

To allow adequate nitrogen removal, 80 m3/h of recirculation flowrate of settled sludge and 120 m3/h of mixer liquor were provided. This determines an HRT of nitrates in the denitrification step ofjust one hour. This value is retained sufficient for a nitrogen removal, up to 86%, only with a denitrification rate greater than 0.2 mg N0 3-N/mg MLVSS d. To reach this goal, the plant was followed over a one year period changing the equalisation management from continuous aeration to anoxic conditions. This was justified by the supposition that settled solids could ferment or hydrolyse, so determining an increase of RBCOD. Two periods with different operative conditions were individuated: a-period (April-May 1993) corresponds to a normal depurative scheme, and b-period (July I993-March 1994) is characteristic of the new approach defined above.

ei, f ac t ory

8

A

A •

&

i~ .. 81

:8

de tr o ~ ltng

pr ocess

8 . p refrymg w estew et er s w ith high oil c oni B 'l -be tt er ed w ast e w a t er s f r om p re t ryi n g

C • re circu lation from slu dg e do wa tering

W as te oi l

o . co nd en se r o f Or l

air ex t r acted fr o m pre ' rying O r . slu d ge ri ci rc u len on tlo w re te 0, 1 - m ix e r liq uo r ricir c u ten cn tl o w re te

Efflue nt

' · A .P .I. se p Ar Rto r

c

Or

z -e queh ee no n bas in 3 ·d en it rifica tor 4 -bi olog i ca l oxrd eno n

5 -s 8conda ry c la rifie r 6 -t hic kening

7 -d ew alen ng 8 - soli d wa st e slu d ge (] - e n o x!c s to p

Fig. 1. BNR plant

JllST l2,9/10-V

296

P.BATnSTONIandG.FAVA

Preliminary results (Battistoni et al.;1994b) obtained following a mass balance of equalisation or measuring the denitrification rate in wastewater feeding the predenitrificator have showed that it is possible to have a sustained increment of RBCOD (17-55%) and a fast denitrification. However, this constitutes only a potential capability of the system that needs a practical verification on the basis of the obtained performances; furthermore the role exerted by the different streams involved in equalisation, together with the real source of the internal carbon production must be attributed.

Plant monitoring Plant monitoring was performed to verify plant performance. Statistical parameters of plant influent and effluent show an average behaviour that requires better details (Tab.2). In actual fact the organic mass loading is almost equal in the two periods with 211 against 193 Kg COD/d during periods a and b respectively, with the COD to TKN ratio of21 and 18. Table 2 - Chemical-physicalcharacteristics ofintluent and effluent wastewater Parameter

Q (mJ/d)

pH

COD (mgIL)

IKN (mgNlL)

NH4 (mgNlL)

Influent

avg min max s.d. avg min max s.d. avg min max s.d. avg min max s.d. avg min max

s.d, avg min max s.d. avg NO z min (mgNlL) max s.d. a period- 24/04/93 - 20/06/93 NO] (mgNIL)

Effluent

a period

b period

622 214 1215 233 7.51 7.40 7.60 0.07 340 215 520 125 16.38 6.17 20.53 6.83 14.82 10.1I 20.46 3.79 0.01 0.08 0.97 0.32

472 160 1060 20S 7.43 6.89 7.85 0.26 409 141 758 187

a period

7.57 7.40 7.70 0.10 24.9 21.6 41.0 2.9 6.2 16.64 5.1 29.87 7.2 4.12 1.0 17.56 3.52 7.54 2.14 26.91 5.68 4.96 1.18 0.26 1.34 0.02 O.OS 4.04 1.62 1.41 0.42 0.08 0.05 0.00 0.04 0040 0.06 0.01 0.10 b period- 21/06/93 - 02103/94

b period

7.30 6.70 7.81 0.33 39.0 20.0 42.0 4.7 4.27 0.61 13.20 2.84 2.30 0.00 7.10 1.82 0.55 0.09 2.30 0.49 0.03 0.00 0.25 0.04

This suggests once more, that the average presence oforganic compounds is enough to satisfy the

Fishprocessing wastewater

297

stoichiometric demand for denitrification (8.6 mg COD for mg N03-N denitrified, as suggested by Ekama et a1.; 1986; and Mamais et al.; 1993) even if all the incoming nitrogen is nitrified in the extended aeration process. For this reason a further contribution ofRBCOD is not needed, but the fluctuation of the amount of fish processed and the different production lines operating during a week must be considered, which makes an equalisation basin with a HRT of 0.63 d useless. Effluents characterizing a and b periods show different nitrogen removal yields with 46% and 79% respectively. This is due both to a better NOx-N and to TKN removal (in fact NOx-N effluent is 8.5% of total influent nitrogen, against only 2.5% in b period; TKN effluent is 38% in a period against the 19% in b). A detailed analysis can be drawn by the graphic behaviour of effluent nitrates (Fig.2) where it is clear how the new management achieves, after a starting period, a lower and constant level of nitrates avoiding the continuous fluctuations due to a lower RBCOD presence observed during a-period.

•... b period

sf'

NITROGEN

mgll

r

r

[;

6~ j

r

t ~

.

2t-

~:.L::::"2.~::!:t:~::d

oI f

110

190

270

350

430

Fig. 2 - Nitrogen in the plant effluent

Internal carbon source Batch experiments were performed on each stream feeding the equalisation and the results compared with the behaviour of the global discharge. However, these data must be considered only as a trend of relative values, since the continuous variation ofcomposition in incoming flows makes it impossible to consider them as absolute value. Waste B, sampled in the effluent of A.P.I. separator (Fig.I), shows a 40% of SCOD as RBCOD with no substantial variations in anoxic conditions up to five days. The behaviour is explained as caused by the vegetable oil present, able to film or inhibit the microrganism growth and avoiding any biological evolution of organic substrates. This means that the waste contributes to the RBCOD of equalisation feeding, but cannot be considered as an internal carbon source ofRBCOD (Fig.3). Influent A+C shows a decreasing trend of the RBCOD to SCOD ratio during the five days of anoxic batch test (Fig.4). This is due to a gradual RBCOD decrease in spite of an increase of SCOD. The physical meaning can be that while some organic solid substrates are solubilized, RBCOD is consumed faster by facultative rnicrorganisms present. This occurence, considered together with the HRT of equalisation, defines a loss ofRBCOD useful for a fast denitrification. Stream B1 was prepared diluting the batter used in the prefrying line one hundred times and discharged at the end of the day, so reproducing a synthetic waste. Anoxic test (Fig.S) demonstrates that the battered wastewaters are the real internal carbon source ofRBCOD. As a matter offact, against a weak increment of

P. BAlTISTONI and G. FA VA

298

SCOD , a much greater increment , up to 72%, of RBCOD is shown in the first day, inspite of their immediate consumption. The global influent of equalisation was simulated adding to A+C influent the battered waste, in a ratio of 300 :1, simulating the real ratio between the incoming streams . Of course the trend observed in the balch test can be modified adopting a different composition (Fig.6). Indeed the sum of the previous plots is observed (Fig.6) , and this is a confirmation that at the HRT of equalisation basin it is possible to increase the RBCOD content of the denitrification feeding utilising battered wastes originated by the prefrying line washing . This effect can also be prolonged to the period when the prefrying line is disabled, by stocking adequate amounts of batter inside the equalisation and avoiding their flotation and loss, with a careful use of the submerged aerator, to guarantee the necessary anoxic conditions . Regarding the biological process determining the increase of the RBCOD , it is possible to conclude that hydrolysis and not fermentation of starchy substances, is the prevalent phenomenon since low to null amounts of volatile fatty acids or lactic acid (lower that 5 mg HAcI1) are observed, while methanol or ethanol were never found . COOl 000 F-;;;::::-::~···::::'~ ~:::::~·:·~ ·:: :::::::~::~·'=:=".·,,:·:=.·.·=::~!, 60 (mgll) 900 rf

• SCOD RBCOD

!

•

800 [.

-

i' 54

.

1 42

L6

;

600 ,!'

J::

500 r

:r,--'

4 00

·t- r··-··-···-~"··-·:·····

o

2

3

5

4

Fig. 3 - COD evolution in anoxic conditions of B effiuent of A.P.I. separator

(m g /l)

.•. , . •

•

, .• ••. •

•

SCOD

•

RBCOD

-e- RBCOD

,

4 50 i

l\_ t

' ___

::·· ·: :·:~::::~·:~::::::·:· 18 7 %

380 .

-

RBCOD 18 2 RBCOD/ s c e D

320

~ 72

290 [ '..........

~ ~ 67

<

260 ! 230

Lo

.

1

, ..

2

3

16 2

•.

4

}

57

5

Fig. 4 - COD evolution in anoxic conditions of A+C influent of equalisator

1

94 %

1

SCOD '"""-.• ..___ -.-···_ ·_··#····1,8 4

j

400 ~

,~

4 10 r · · ·..• ..;:.:.:=:·:~~·:~::~:

TIME (d)

TIME ld)

coo 5°o rf\~-~' ~ r "

~~II

RBCOD/S C1D4 8

700 t

f

%

~

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.) 6 4

30 0 .

~

r

260 ~ ,

o

I

L

1

2 3 TIME ldl

;,:

A •• • __

4

64

6

Fig. 5 • COD evolution in anoxic conditions ofB 1 influent of equalisator

Fig. 6 - COD evolution in anoxic conditions of global influent the treatment plant

Fish processingwastewater

299

Best operative conditions Best management was also sought in the direction to improve the biological sludge sedimentation, without changing sludge stability. A gradual lowering of the MLSS in the activated sludge process was achieved through sustained dewatering. Different and opposite behaviours could however be registered over the one year observation. A biomass content in activated sludge lower than 3.4 Kg MLSS/m 3 was reached only in 1994 (Fig.7). This situation was determined by the poor thickening of sludge with partial use of the solid bowl machine. The mathematical model previously applied for transient conditions (Battistoni et a1.;1994a) was utilised to follow the sludge retention time (SRT) on the basis of the amount of solid sludge daily wasted and the biomass content in the activated process (eq. 1-2). TVS S x V x••--•••• = Pxt - Pxexp TS

(1)

XxV SRT =

(2)

.------

Pxt

Pxt Pxexp X S V

stoichiometric waste sludge experimental waste sludge MLVSS at the beginning of transient state slope ofMLSS trend volume of biological reactor

(Kg TVS/d) (Kg TVS/d) (Kg/m 3 )

(mg MLSS /I d) (m 3)

The MLSS graphical pattern observed in three sub-periods (Fig.7 ) shows an SRT decreasing trend from 62 to 34 days (Tab.3). This causes a double effect: the variation of specific oxygen uptake rate (SOUR) (Fig.S) and sludge settling velocity (Fig.9). Higher SOUR with younger sludges is obtained and the waste sludge remains stabilized (SOUR lower than 10 mg Ozlmg MLVSS h at an SRT of 34 d). A greater settling velocity is gradually obtained by changing SRT. The evaluation was done at three different sludge concentrations: 2000, 3000, 4000 mgll, respectively equal to the design data, 1994 and 1993 operative conditions (Fig.9). Comparing the settling velocity with operative overflow rates of the final sedimentation tank, the two working periods can easily be differentiated .

\ i

\

3800 t

::~~ " " 110

190

270

Tim. ld )

\

80

I

3

r

•.

360

430

Fig. 7 • MLSS and cumulative wasted sludge

P. BAlTISTONI and G. FAVA

300

Table 3 - SRT of activated sludge process Subperiod

Slope

R

Pxt

Pxexp

SRT

MLVSS

d

gMLSS/mJd

Pearson

KgTVS/d

KgTVS/d

d

mgIL

Coefficient 1 - 147-208

-14.07

0.80

48.14

57.82

62

3740

2 - 244-365

8.19

0.87

50.47

44.84

52

JJ II

J - 370-426

4.82

0.70

62.31

58.99

34

2645

Particularly in the b period the overflow rate is always lower than the settling velocity, so suggesting the possibility to practically operate at an overflow rate double that usually adopted (OF= 6 m J/m 2 d corresponding to 950 m3/d offlowrate discharged). 16 ~ J SOU R , (mg 02 I , g VSS

~ /.

r·· ·.·

··r···· ········ . '.

. ·.. ,·'.

~·

t

~

~

~

.;\1i

\ '\ ,

1

4

,

Sell ling

•

+

Ve loc ity

I

.

20 ,

!1

"

i

..

"'.:.

i i 10 .

OR -3m /d

,":.

b p e r iod ...

I

/ .... . !

O R - 4m /d

/"

I

~

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•

; ;

•,

Im /d l

/ i.····.... ,

"f '

15 .

!~

"

.

MLS S - 2000 mg l L ML SS - 3000 mg lL ML SS _ 4 0 00 mg l L

5 ' •

[

ptirlO ~

". ".. . . . ",

~

t

• !i

\I ;!

+;... ; ~...i----+rJ-.-+J..

I ....

o l~~':::='.~ :::: . 190

270 TIME Idl

35 0

Fig. 8 - SOUR in activated sludge

4 30

1 10

190

2 70

350

4 30

TIME (dl

Fig. 9 - Sludge settling velocity and overflow rate (OF)

Plant reliability Unusual altered conditions, i.e. damage of the electromechanical systems, can determine major impairments in the effluent standard conformity because of the new scheme adopted . This has been observed during two events which happened in 1994: l-the break down of the level control of the pump used to feed the denitrificator, 2-the mixer inside the predenitrificator. The first determined both emptying of the equalisator with an increment ofMLSS up to 30%, in the oxidation process (Fig.l 0) and the development at some time of a shock loading of volatile suspended solids. The consequences were an initial increase of nitrates since no more RBCOD was generated, then a block of nitrification probably due to insufficient oxygen availability; finally a gradual new start-up of the nitrification and denitrification processes. All these events required about 25 days to restore them to a the normal condition. The second lasted ten days (Fig.I I A-B time), determining an accumulation of the biomass inside the denitrification and a decrease in the activated sludge process. This produced a high feed to rnicrorganisms ratio with the consequent loss of nitrification. After removing the damage (point B Fig. 11) a further ten days were necessary to restore normal condition. In conclusion, the two case histories show that only the first determines major risks with respect to a traditional process, however risks are overcome by the advantages determined with the RBCOD production.

Fish processing wastewater ........... ......1'

_ ••••••":'. _

I

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lV

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~~~y~ '~ .! : C::S mgl ./'\

.

•

16 10 '

+ N03 mgNll N02 mgNll MlSS mg ll

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o --" 66

'

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NH4 mgN Il

i'.

6'

····9 6000

- ··T···· ··-

'!'

.,~.

'13000 1

\

73

82

91 TIME (dey 01 the yeo, 1994)

t

1f(' t'I~~I; A

25 1·

mall t.

f

j

15 r

lo r r

100

Fig. 10 -Activated sludge monitoring during break down of the pump feeding the predenitrificator

;1

20 F"'e!i~·

1

~ 2000

!

. I I ilooo -"*1 1 ,.;.",~~ _.•~;jo 64

30 """""'- '

.

JB

\

' /

t I

5000

MlSS

'......._~ .. . 4~~:/I) ---.-c.

\

lJI •.

i .~

f

5.

- .-,..-- ,

'~I I{ \

;

t. ,

, 1

301

1000

/

rr:.. . . . ,

o k'..:~~!1c:~~f:!=j~~~ 94

102

110

118

126

T IME (d oy 0 1 the yeo, 19941

Fig. 11 -Activated sludge monitoring during break down of the mixer in the predenitrificator

CONCLUSIONS Higher NOXoN or TKN removal are obtained producing RBCOD by internal carbon source. The hydrolysis of starchy substances is the main route to obtain RBCOD from battered wastewaters via anoxic equalization. Best operative conditions are found at lower SRT, about 30 d, enough to produce a stabilized sludge and a high quality effluent, with good settling velocity of activated sludge. Plant reliability was observed during the whole period of observation, break down of the electromechanic systems required an extra 25 days to restore high removal yield.

ACKNOWLEDGEMENT The study was carried out with the collaboration of Ancoopesca S.p.A Ancona, Italy. We are also grateful to Pierpaolo Piantini and Serena Stella for their technical assistance. The research was supported by a Ministry of The University and ScientificlTechnological Research Grant (National Research Project) .

REFERENCES Aguiar. AL.e., and Sant Anna Jr., G.L. (1988) . Liquid effluents of the fish canning industries of Rio de Janeiro State , Treatment alternatives. Environ. Tech. Lett.. 9, 421-428 . APHA ( 1985). Standard methods for the examination of water and wastewater 16th edition. American ~ Health Association, Washington D.C. Battistoni,P.,Fava,G ., and Gatto,A.(1992a).Fish processing wastewater: emission factors and high load trickling filters evaluation.Wat . Sci Tech.. 2S(), 1-8. Battistoni,P. , Fava,G., and Gatto,A. (1992b) . High load trickling filters accumulation and sludge production Wat Sci Tech. 26(9-1 2405-2408 . Battistoni, P. and Fava,(l994a)G., Fish processing wastewater. Treatment requirements by line production changes . Wat. Sci. Tech .. 29C9l. 111-119.

u

302

P. BArnSTONI and G. FAVA

Battistoni,P ., Fava,G., (1994b) Fish processing wastewater. Denitrification improving through internal production of readily biodegradable COD, Preprint Water Quality International 1994, IAWQ 17th Biennial Intern. Conf., Budapest, Hungary, 24-29 July Ekama,G.A., Dold,P.L., and Marais,G. v. R. (1986) . Procedures for determining influent COD fractions and the maximum specific growth rate ofheterotrophs in activated sludge systems. Wat. Sci. Tech ., 18, f.Q1 91-114. Ekama, GA, Clayton,JA, Wentzel, M.C., Marais , G.v.R. (1991). Denitrification Kinetics in biological nitrogen and phosphorus removal activated sludge systems treating municipal waste waters. Wat. Sci, ilm., 23 (4-6) , 1025-1035. Henze M, (1991). Capabilities of biological nitrogen removal processes from wastewater. Wat. Sci. Tech.. 23, (4-6), 669-679. Lotter,L.H., and Pitman, A.R. (1992). Improved biological phosphorous removal resulting from the enrichment of reactor feed with fermentation products. Wat. Sci. Tech .,26(5-6), 943-953. Kristensen,G.H., Joergensen,P.E., Strube,R:, and Henze,M. (1992). Combined pre-precipitation, biological sludge hydrolysis and nitrogen reduction - a pilot demonstration of integrated nutrient removal. Wat. Sci. Tech. 26(5-6),1057-1066. Mamais,D., Jenkins,D ., and Pitt.P (1993) . A rapid physical-chemical method for the determination of readily biodegradable soluble COD in municipal wastewater. Wat. Res . 27(1), 195-197. Mendez, R., Omil, F., Soto, M. and Lema, J.M. (1992) . Pilot plant studies on the anaerobic treatment of different wastewaters from a fish-canning factory. Wat. Sci Tech. 25 (), 37- 44.