BIOCHEMICAL MODELING OF PAN AFRICAN PAPER MILLS AERATED LAGOONS, WEBUYE, WESTERN KENYA

ABSTRACT

Pan African Paper Mill (PPM) in Webuye, Kenya has a production of 120,000 tones of paper per year. Expansion programs at the mill over the last 27 years have resulted in an increased effluent load on the wastewater treatment system originally designed to handle only 25,000 m³/day. The increased loading of up to 40,000 m³/day, has often led to the violation of the new Environmental Management and Co-ordination Act, of 1999 (EMC Act), which outlaws all forms of pollution. A research project was carried out for six months to establish a biochemical model of the aerated lagoons at PPM that can be used as a tool to predict and evaluate the lagoon performance under varying wastewater characteristics and environmental and physical conditions. A laboratory experiment using a bench scale reactor and the effluent from the mill primary clarifier overflow (PCO) was first used to determine the effluent kinetic coefficients (major model inputs). These coefficients i.e. maximum growth yield (Y), maximum rate of substrate utilization (k), microbial decay coefficient (K_d), half substrate saturation constants (K_s) and maximum specific growth rate (µ_m), had values of 0.465 mg/mg, 2.0 day^-1, 0.05 day^-1, 51.5 mg/l, and 0.93 day-1 respectively. These kinetic coefficients and the wastewater pollution parameters were then used as inputs in the 12 first order differential equations that made up the biochemical model. Second order Runge-Kutta method and a Fortran 90-computer package was subsequently used to solve these equations simultaneously. The validation of the model at 95% confidence level for wastewater parameters: COD, DO, VSS, organic phosphate, nitrate, ammonia, alkalinity, detritus, algal biomass, nitrite, inorganic carbon, and inorganic phosphate yielded a correlation coefficient (R) of above 0.9. These levels of R shows that the model generated can be used to monitor and predict the performance of the aerated lagoon.

The influent and effluent results showed that the mill effluent COD, BOD₅, TS and colour at the effluent discharge point into the river were high and did not often comply with the standards provided under the EMC Act, Water Act, Public health Act and Kenya Local Authority Act. The level of pH, temperature and alkalinity were, however, within the limits required for effective operation of an aerated lagoon. The phosphorus, nitrate and nitrite levels in the aerated lagoon were not sufficient as essential nutrients for bacterial metabolism of organic matter while the level of dissolved oxygen was less than 1 mg/l in spite of the aeration. The effluent was found to be highly coloured at all sampling points. The biochemical model thus established could be used as a cost effective to determine the level of aeration needed and the artificial bacterial culture required for the degradation of the paper mill effluent to an acceptable level.

INTRODUCTION

Pan African Paper Mills (PPM) in Webuye Kenya was originally designed, established and commissioned in 1974 to produce 45,000 tones of paper per annum. The mill utilizes Kraft and mechanical pulping processes to produce industrial and cultural grades of paper using Eucalyptus saligna, Pinus patula, Pinus radiata and Cupressus lusitanica species (KFMP, 1994; Greennews, 2001). In 1992 the mill's production grew to 96,000 tones per annum and by the year 2001, the production had reached 120,000 tones per annum. Over the years, these expansions have increased demand for process water leading to the discharge of large wastewater volumes of about 40,000 m³/day into a treatment system originally designed to handle only 25,000 m³/day of effluent (Mashall and Germain, 1979; Greennews, 2000). At PPM, the mill's effluent from various sections within the factory is collected in underground pipelines and conveyed by gravity to the treatment plant. This combined mill effluent is passed through trash screens to remove suspended solids before it is discharged into primary and secondary clarifiers to enable the settling of suspended solids into sludge. The overflow from the clarifiers is then taken to the aerated lagoon. In the aerated lagoon, oxygen is supplied by surface 5 surface aerators, each rated at 2.6 MWh, and two air blowers to promote the effective growth and accumulation of microorganisms needed for the biodegradation of the organic material in the effluent. Finally the effluent is discharged into polishing lagoons, before being discharged into Nzoia River through diffusers (Kahuki, 1988; Greennews, 2001).

Before the overflow from the primary clarifier goes into the first aerated lagoon, it is mixed with a "microstater 110", a bacterial culture imported from the U.S.A to help degrade the organic material in the effluent. PPM must also add a steady solution of diammonium phosphate (DAP) to make up for the deficiency of N and P and maintain the following parameters BOD₅:N:P within the appropriate range of 100:5: 1. An adequate detention time, an optimum presence of nutrient and microorganisms and an adequate aeration rate should, in theory, insure that PPM complies with the effluent discharge standards such as a BOD₅ of 80 mg/l, volatile suspended solids of 60 mg/l, free ammonia of 6 mg/l and a pH range of 6 - 9. However fluctuations in the pulp and paper production levels and in the mill effluent strength can, from time to time, upset the wastewater treatment system, which, under normal circumstances should otherwise yield good quality treated effluent. Periodic checks are therefore necessary on some of these parameters for the proper operation of PPM effluent treatment system.

The increased load on the effluent treatment system has significantly reduced the efficiency of the aerated lagoons thus partially treated mill effluent has been reported in Nzoia River (Achoka, 1998). Such discharges will, for instance, reduce the transparency of water. Consequently, the poor water quality is likely to severely affect the fauna, flora as well as the health of the adjacent human population, which will in turn affect the bio-diversity of Nzoia River discharging into Lake Victoria, the second largest fresh water lake in the World. The effluent treatment system is illustrated in Figure 1.

The objective of this study was to (a) determine the level of pH, temperature, DO, BOD₅, COD, solids, nitrites, nitrates, alkalinity, phosphates, algae mass, inorganic carbon and detritus mass at the entry and exit of the aerated lagoons (b) determine the major kinetic coefficients for PPM aerated lagoon such as maximum growth yield (Y), maximum rate of substrate utilization (k), microbial decay coefficient (K_d), half substrate saturation constants (K_s), maximum specific growth rate (µ_m) (c) establish and calibrate a biochemical mathematical model by use of the pollution parameters in (a) and (b) as model inputs and material balance equation around PPM aerated lagoon.

MATERIALS AND METHODS

Model Development Assumptions

The individual aerated lagoon units were considered as biological continuous flow mixed reactors and material balance theory was applied around them.

The Model

The model developed in this study is a biochemical model, which, in its general form can be written in time differential form as:

Mass balances of the form of equation 1, for continuous flow completely mixed reactors were written for each biochemical component modeled. The biochemical model was thus made up of twelve ordinary, first order, nonlinear differential equations for soluble chemical oxygen demand, dissolved oxygen, bacterial mass, algal mass, inorganic carbon, organic and inorganic phosphorus, organic nitrogen, ammonia, nitrate and alkalinity. The following 12 equations developed around PPM aerated lagoon were adapted from Ferrara and Harleman (1980) and Fritz (1985):

Substrate mass balance

Heterotropic bacterial mass balance

Organic phosphorus mass balance

Organic Nitrogen Mass balance

Oxygen generation and utilization

Algal growth balance

7. Ammonia mass balance

9. Alkalinity mass balance

10. Inorganic Phosphorus mass balance

11. Total Inorganic Carbon generation & Utilization

12. Detritus mass balance

Runge–Kutta Method and Model Program

The twelve linear differential equations were solved by second order Runge–Kutta integration technique to yield the time development concentration of each component. This method is widely used for solving a system of differential equations with a high degree of accuracy. It uses several intermediate calculations at each stage which increase the order of accuracy (Nakamuru, 1991). The second order Runge- Kutta method was also chosen because of its simplicity in programming, low truncation errors, fast convergence and the requirement for only initial conditions for S_o, X_bo, DO_o, P_o, N_o, NH_4o, Alk_o, D_mo, X_ao, N_oi, Ct_io and P_io (Fritz, 1985, Barlow and Barnet, 1998)

Fortran 90 program was used to solve the 12 first order differential equations simultaneously. The suitable step increments in the program were determined from simulation runs using various intervals allowing for uniform iterations comprising 50 steps, until the computed results were within the desired degree of accuracy or 5% of the expected value. The iteration step size (h) was determined according to the method suggested by Stroud (1996) i.e. h=(B-A)/n

Where B – Highest level of parameter

A – Lowest level of parameter

n – Number of steps

The negative time constant was avoided because it would make the Runge–Kutta method unstable (Nakamura, 1991). The most appropriate step intervals were 0.01 mg/l, 0.025 mg/l, 0.001 mg/l, 0.001 mg/l, 0.0001 mg/l, 0.001 mg/l, 0.01 mg/l, 0.05 mg/l, 0.001 mg/l, 0.05 mg/l, 0.05 and 0.025 mg/l for COD, VSS, P, N, DO, X_a, NH₄ , NO₃, P_oi, C_ti, Alk and D_m mass respectively.

Model Validation

The testing of the adequacy of the model was done using field data and values predicted by the model. The field data included values for COD, dissolved oxygen, volatile suspended solids, organic phosphorus, nitrite, nitrate, ammonia, alkalinity, detritus, algal mass, inorganic carbon and inorganic phosphorus. Statistical regression analysis at 95% confidence level was done on the observed and predicted values for the determination of model validity.

Major Kinetic Coefficients Included in the Model

The solution of the differential equations written above was dependent of some kinetic coefficients that are effluent specific. These coefficients were obtained by operating a bench scale reactor using effluent from the entry point to first aerated lagoon, according to the procedure described by Metcalf and Eddy (1979). The reactor configuration consisted of an aeration basin of 80 litres capacity, and divided into an aeration chamber and a settling chamber by an adjustable glass baffle (Figure 2). The reactor was set up in such a manner that allowed gravity flow of the wastewater into and out of the system, by use of rubber tubing.

Aeration was done through compressed air bubbled by an electric air pump through the bottom of aeration chamber. The reactor was stirred continuously and wall growth was regularly suspended. The reactor water jacket was kept at 25^o C in the laboratory and 150 g of DAP was added as nutrients to satisfy the requirements of the growing bacteria based on theoretical proportions of nitrogen and phosphorus in biomass (Metcalf and Eddy, 1979). One hundred grams of commercial bacteria (Microstarter 110 bacteria) used for the biodegradation PPM effluent was also added to the reactor.

The reactor was operated for hydraulic retention time ranging from 1 to 30 days. Steady - state conditions (volume, aeration rate and flow rate) were maintained for each retention period. Wastewater samples were allowed to flow into the reactor maintained at a constant continuous flow. Detritus material and sludge were removed from time to time to avoid any interference with the value of effluent COD. Substrate concentrations injected into the reactor and operated at various hydraulic retention times were sampled from the reactor inlet and outlet and analyzed specifically for COD (S_o) and VSS (X_o). These values were then used for determination of biochemical constants necessary for aerated lagoon modeling.

Figure 2. Bench Scale Continuous Flow Stirred Tank Reactor without Recycle

Sampling, Sample Preservation and Analysis

Sample Collection

The procedure followed in the collection, handling and analysis of wastewater samples was adapted from APHA (1989) with some minor modifications. The sampling points were at the Primary Clarifier Overflow (PCO), Pond One Discharge (P1D), River Discharge (RD) as well as Pond Two (P2D) and Pond Three Discharge Points (P3D) as illustrated in Figure 1. Since the flow and consistency of the effluent vary throughout the day and with rainfall, a series of grab samples were collected randomly in the morning, midday and late in the evening over a period of six months. Appropriate quantities of samples were drawn by means of a calibrated sampling can attached to 1.5 m cord and weighted at one end.

A variety of methods and procedures routinely used in the analysis of natural waters and wastewater were used. These were adopted from APHA (1989), Beveridge et al (1985), Boyd (1973), Stirling (1985) and Boyd and Tucker (1992). The parameters analyzed were biochemical oxygen demand, chemical oxygen demand, total solids, total volatile solids, total suspended solids, volatile suspended solids, electrical conductivity, dissolved oxygen, nitrates, nitrites, total phosphorus, colour, ammonia and alkalinity

RESULTS AND DISCUSSION

Waste Water Characteristics

Table 2 below presents the mean levels of physicochemical parameters at sampling sites i.e. primary clarifier overflow (PCO), pond one discharge (P1D) and River discharge (RD) that were used in calibrating the biochemical model.

Table 2. The mean levels of physicochemical parameters in the wastewater and the River discharge point at Pan African Paper Mills.

PH

The mean pH for PCO was found to be 9.25, however the dilution effect of lagoon wastewater reduces this pH to an average of pH 7.7 at P1D as well as RD. The range of pH variation fell between 7.23 and 10.71, 6.92 and 8.52 and 7 and 8.1 for PCO, P1D and RD respectively, while that of the aerated lagoon ranged between 6.8 and 8.4. The pH of the effluent from the aerated lagoon was found to be within the accepted range for biological treatment systems i.e. not less than 6 and not more than 8.5 (ECSC et al, 1994). Effective pH for most biological oxidation systems cover the range of pH 5 to 9 with the optimum rates occurring over the range of pH 6.5 to 8.5. This range is considered normal for most fresh water fish, bottom dwelling invertebrates and microorganisms (US - EPA, 1986).

Temperature

It was found that the temperature of the aerated lagoon varies between 20.5^o C and 33^o C, that of PCO varies between 38^o C and 41^o C, while for P1D and RD the range was 27^o C to 33^o C, and 18.5^o C to 22.5^o C respectively. These levels of temperature are considered conducive for the biodegradation of dissolved organic substances thereby helping to meet the set standards (Ekenfelder, 1989).

Nutrient Levels

It was found that the level of total phosphorus, nitrate and nitrite at PCO was 0.0055 ± 0 .0035 mg/l, 0.05 ± 0.05 mg/l and 0.02 ± 0.02 mg/l respectively. At P1D the Total phosphate, nitrate and nitrate were 0.0305 ± 0.0295 mg/l, 0.0315 ± 0.0285 mg/l and 0.1 ± 0.1 mg/l respectively. At the discharge point into the River, the total phosphorus, nitrate and nitrite were 0.0095 ± 0.0005 mg/l, 0.004 ± 0.003 mg/l, 0.0255 ± 0.0245 mg/l. These levels of essential nutrients necessary for bacterial metabolism of organic matter were not sufficient. They were, however, similar to the levels found for other pulp and paper effluents hence the need to bring in supplemental nitrogen and phosphorus from external sources (Eckenfelder, 1989: UNEP, 1981; Struthridge, 2000). Depending on solid retention time, temperature of the pond and the growth phase of the microorganisms, high rate aerated biological treatment should be operated at BOD₅: Nitrogen : Phosphorus ratio of 100: 5 : 1 ( UNEP, 1981; Helmers et al, 1951).

Effluent Colour

The colour of PPM effluent was found to be high at all sampling points i.e. at PCO, aerated lagoon, P1D as well as RD. The colour ranged from 700^oH to 4000^oH, 500^oH to 2125^oH, and 700^oH to 3600^oH for PCO, P1D and RD respectively. The mean values showed an increase in colour from 1280^oH for PCO, 1478^oH for P1D and 1500^oH for RD. The mean colour of the discharge point into the River was 1500^oH which was significantly above the recommended range of 50^oH to 200^oH (UNEP, 1981) and the Local government effluent standard of 5^oH. The effect of coloured pulp mill effluent results in the suppression of primary production (Howard and Walden, 1965). Since oxygen generation by photosynthesis depends on light penetration, highly coloured wastewater such as pulp and paper cannot be treated by natural activities such as algae photosynthesis (Eckenfelder, 1989).

Besides causing biological effects, colouring compounds have been noted to be toxic and aesthetically repulsive (UNEP, 1981).

Dissolved Oxygen

Dissolved oxygen concentrations in the PPM aerated lagoon, PCO and P1D were less than 1.0 mg/l. At the discharge point into the River Nzoia, dissolved oxygen was between 0.0 mg/l to 1.0 mg/l with a mean of 0.365 mg/l. Low levels of oxygen in the PPM aerated lagoon indicate that there was not sufficient aeration and that we might have been dealing basically with anaerobic conditions. These anaerobic action results in the release of free gases like carbon dioxide, hydrogen sulphite, and ammonia, which cause a foul smell around the lagoons (Lander et al, 1977).

Dissolved oxygen in an effluent treatment system is an indication that very significant oxidation has been achieved by the treatment employed. This low oxygen level in the lagoon could also be attributed to the presence of fibres and fibre particles from pulping and papermaking, which settle at the bottom of the lagoon in form of fibre banks in which fermentation may occur causing oxygen depletion (Lander et al, 1977; Johnson et al, 1996). These high load of oxygen demand organic compounds is a common characteristic of wastewater from Kraft pulp and paper mills (ECSC – ES – EAEC, 1994; Etiegni, 1994). There is therefore need to have a routine emptying of the lagoon to rid it of these anaerobic processes.

Biochemical Oxygen Demand (BOD₅)

The results of BOD₅ for PCO ranged from 120 mg/l to 240 mg/l with a mean of 234.7 mg/l while that of P1D ranged from 130 mg/l to 235 mg/l with a mean of 171 mg/l hence significantly lower than PCO value. The RD, BOD₅ value ranged from 60 mg/l to 150 mg/l with a mean of 118 mg/l. The latter values are significantly high and above the local authority standard and the World Bank standard of a trade effluent of 80 mg/l and 50 mg/l respectively. The high BOD₅ level could be attributed to the high strength of influent to the lagoon as well as internal source from the benthic deposits, which diffuse soluble BOD₅ into the water. The BOD₅ design value to meet the effluent discharge requirements by the Local authority Act had initially been set at: influent BOD₅ load of 360 mg/l for PCO, 90 mg/l for P1D, and effluent BOD₅ of 23 mg/l for Discharge into the river based on a maximum flow rate of 25,000 m³/day of combined pulp and paper mill effluent. However, present mill operations indicate the effluent rate has increased to 40,000 m³/day thereby straining the effluent treatment system and reducing the retention time from the initial designed value of 7.4 days to approximately 5.5 days.

Chemical Oxygen Demand (COD)

The results of the study showed that the COD value for PCO (572 ± 256 mg/l ) was significantly higher than P1D (490 ± 290 mg/l) and RD (122.5 ± 77.5 mg/l). In a pulp and paper effluent COD is mainly contributed by fibres, high molecular substances of lignin and carbohydrate, black liquor and liquor condensates from Kraft mill department (Landner et al, 1977).

Solids

The results for total solids for PCO was (1030 ± 930 mg/l), P1D (1475 ± 1305 mg/l) and RD (330 ± 130 mg/l) with mean value of PCO (872.35 mg/l), P1D (970.87 mg/l) and RD (333.37 mg/l). The TSS value were (250 ± 150 mg/l) with mean of 212.45 mg/l, (435 ± 205 mg/l) with mean of 486.21 mg/l and (105 ±45 mg/l) with a mean of 94.56 mg/l for PCO, P1D and RD respectively. The values for TSS at the discharge point into the river were found to be above the allowable limit of 25 mg/l, which is harmful to fish life. The major part of suspended solids in PPM effluent consisted of fibres or fibre particles.

Alkalinity

The results for alkalinity ( 355± 1135 mg/l as CaCO₃) for PCO, (300 ± 180mg/l CaCO₃) for P1D of 400 ± 220 mg/l as for aerated lagoon are all below the adequate buffer capacity of 2000 mg/l and this could explain why the aerated lagoon does not function satisfactorily as there is no adequate pH control.

Kinetic Coefficients

Figure 5. Kinetic Coefficients K_s and k for first Aerated Lagoon.

The value of K_s for PPM aerated lagoon was 51.466 mg/l as shown above. These values fall within the range of 25-70 mg/l reported Metcalf and Eddy (1979). This was however significantly different from values for domestic sewage and soybean processing factory of 120 mg/l and 355 mg/l respectively (Jorden et al, 1971). The value was however not significantly different from the New Mexico aerated lagoon treating municipal and high strength animal waste found to be 50.0 mg/l (Larsen, 1974). The low K_s value for microorganisms growing in PPM aerated lagoon indicates that its effluent is not easily biodegradable as compared with the municipal wastewater (Slade et al, 1991).

For PPM aerated lagoon the value for k was found to be 2.0 day^-1, which is significantly lower than the value of 3.125 day^-1for aerated lagoon treating bleached Kraft mill effluent at the New Zealand Forest Products aerated lagoons for instance (Slade et al, 1991). These values are lower than that of municipal wastewater with k between 8 and 10 day^-1 (Loehr, 1984). This implies that there is a low rate of substrate utilization in the effluent from PPM which could be attributed to low concentration of readily biodegradable organic matter found in most domestic sewage or municipal wastewater (Ekenfelder, 1989).

The values for coefficients Y and K_d were determined by plotting the term (1/q) versus (So -S)/Xq. The y intercept equals (-K_d) while the slope of the curve equals Y, as illustrated in Figure 6 below. The value of the coefficient m_m was determined using equation (m_m =kY). These gave the value of K_d = 0.05 d^-1, Y= 0.465 mg/mg and m_m = 0.93 d^-1.

Figure 6. Kinetic Coefficients Y, _m and K_d for first Aerated Lagoon

The value of Y for PPM aerated lagoon was found to be 0.465 mg/mg as shown in Figure 4. This indicates that the growth yield of heterotrophic microorganisms growing in the first aerated lagoon was Y = 0.465 mg cell COD (mg COD)^-1. This was found to be not significantly different (P > 0.05) from values by Slade et al. (1991) on aerated lagoon treating bleached Kraft mill effluents with Y=0.51 mg/mg. For domestic sewage Y=0.74 mg cell COD (mg COD)^-1, while soy bean processing shows a Y value of 1.036 mg. VSS. (mg COD)^-1 (Jorden et al, 1971). Typical Y value for the activated sludge process is 0.6 mg VSS. (COD)^-1 (Rich, 1982 a).

The value of microbial decay coefficient (K_d) for PPM aerated lagoon was 0.05 day^-1, as shown in Figure 4. This value is similar to a decay coefficient for aerated lagoon treating Kraft mill effluent of 0.054 day^-1 (Slade et al, 1991). The value for PPM aerated lagoon is also within the range for values reported in the literature with a typical K_d between 0.05 to 0.1 day^-1 (Thimuruthi 1974; Metcalf and Eddy 1979; Benefield and Randal, 1980). This parameter is associated with the activity of the microorganisms themselves, rather than wastewater specifically. The value of the specific decay rate, K_d, appears to be relatively independent of the wastewater being treated ( Rich, 1982a).

The value of m_m for PPM aerated lagoon was 0.93 day^-1using equation 20. This value is similar to values determined for aerated lagoons treating bleached Kraft mill effluent of between 0.71 to 0.95 day^-1 with a mean of 0.85 day^-1 (Slade et al, 1991). If K_s is low and m_m is high, it is possible to achieve efficient COD removal at low substrate concentration (Chudoba et al, 1985; Slade et al, 1991). Values obtained was low as compared to values for domestic sewage of 13.2 day^-1, pork processing of 17.3 day^-1and Soybean processing 12.0 day^-1(Jorden et al, 1971).

The low value of m_m and the high value of K_s indicate that the microorganisms growing in aerated lagoon have a low growth rate due to the low amount of readily biodegradable substrate (Chudoba et al, 1991). As m_m is a specific growth rate, increasing the mass of microorganisms within the lagoon could increase the overall growth rate and thus the COD removal rate. This could be best achieved by recycling sludge or increasing the dosage of "Microstarter 110", which is normally added to the effluent entering the aerated lagoon from PPM factory.

Physical Constants and Environmental Conditions

The physical constants and environmental conditions were monitored and recorded during the research period as listed in table 4 below. The build up of biochemical model and its use require a knowledge of both the constants and environmental conditions prevailing around PPM, since they influence the performance of the aerated lagoons (Jorgensen et al, 1985).

Biochemical Constants

Other biochemical constants were borrowed from literature as indicated in Table 5. These biochemical constants are required for effective application of a biochemical model (Gromiec et al, 1983; Metcalf and Eddy, 1979; Fritz, 1985).

Runge–Kutta Method and Model Program

The programme output showed that above the 50^th loop the results became divergent and hence should be avoided. The programme was therefore set to run for only 50 loops.

Model Validation

The testing of the adequacy of the model was done using field data and values predicted by the model. The field data included values for COD, dissolved oxygen, volatile suspended solids, organic phosphorus, nitrite, nitrate, ammonia, alkalinity, detritus, algal mass, inorganic carbon and inorganic phosphorus. Statistical regression analysis at 95% confidence level on the observed and predicted values for the determination of model validity gave coefficient of determination (R) values for COD, DO, VSS, P_o, N_o, NH₄, Alk, Dm, X_a, D_m, X_ai, C_ti and P_i were 0.9656, 0.9641, 0.9754, 0.9754, 0.9921, 0.9616, 0.9661, 0.9859, 0.9969, 0.9621, 0.98216 and 0.9777 respectively. These validation results show that the model gives a reasonable good approximation of the field data and hence can be used by PPM as a predictive tool for monitoring the performance of the aerated lagoon.

Table 4. Physical constants and environmental conditions for PPM aerated lagoon.

Sources:

• 1. Ministry of water development, Bungoma.
• 2. Ministry of Agriculture & livestock development, Webuye.
• 3. PPM weather station.
• 4. Greennews, 2001
• 5. * Determined at PPM laboratory

Table 5. The values for Biochemical constants for aerated lagoons.

* Determined for the bench – scale laboratory experiment

CONCLUSION AND RECOMMENDATIONS

This study has shown that a satisfactory biochemical model can be developed to simulate the biochemical and biomass reactions occurring within an aerated lagoon treating pulp and paper mill effluent. The high degree of correlation coefficient of at least 95% indicates that the mill's effluent parameters can be reasonably accurately predicted.

This biochemical model should help PPM realize considerable savings by not setting up lengthy and expensive bench scale experiments aimed at improving the operation overall wastewater treatment system. PPM, for instance, could use this biochemical model, to determine the appropriate amount of "microstarter 110" to be added to the effluent by manipulating the bacterial mass balance equation or the adequate amount of essential nutrients needed by varying level of phosphate and nitrate mass balances. The level of aeration hence oxygen needed to achieve a certain degree of wastewater treatment can also be determined through this biochemical model by the manipulating of oxygen mass balance equations.

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