TREATMENT OF INDUSTRIAL WASTEWATERS BY ANAEROBIC DIGESTION

AUTHOR: NWEKE, CHINENYENWA NKEIRUKA

DEPARTMENT: CHEMICAL ENGINEERING

AFFILIATION: NNAMDI AZIKIWE UNIVERSITY, AWKA

This work is aimed at studying the treatment of vegetable oil, soft drink and slaughterhouse industrial wastewaters by anaerobic digestion. The digestion period lasted between 30-50 days. The maximum volume of gas production by downward displacement of water for vegetable oil wastewater (VOW), soft drink wastewater (SDW) and slaughterhouse wastewater (SHW) were 35ml, 25ml and 150ml respectively. Microorganisms grew as temperature increased from 28 to 32oC for VOW digestion, 27 to 31oC for SDW digestion and 27.9 to 28.2oC for SHW digestion after which the growth started decreasing. The kinetic parameters which were maximum rate of substrate utilization (K), half velocity or saturation constant (KS), endogenous decay coefficient (Kd), biomass or microbial growth yeild (Y) and maximum specific microorganism growth rate (μmax) determined for VOW digestion were determined as 0.8215 day-1, 163.31mg/l, 0.0838 day-1, 0.2813mg/mg and 0.231day-1 respectively. These values indicate that practical digesters for VOW will require inoculation of the feed with microorganism to increase the rate of digestion. SDW kinetic parameter results for K, KS, Kd, Y and μmax were calculated as 0.350day-1, 204.12mg/l, 0.0222day-1, 0.194 and μmax 0.068day-1 respectively with the value of K indicating that inoculation will be required for its digestion rate to increase. The kinetic parameter results for SHW digestion for K, KS, Kd, Y and μmax were evaluated as 31.847day-1, 25,347.77 mg/l, 0.009day-1, 0.034 mg/mg and 1.073day-1 respectively. The value of K obtained was higher than that of VOW and SDW digestion. This showed that digestion proceeded at a high rate and did not need inoculation for biological treatment. The kinetics of anaerobic digestion of the wastewaters were all described by first order kinetic model. The net specific growth rates of microorganisms (μnet) for the wastewaters also were observed to reduce with time. Simulations of kinetic model for the design of continuous reactor for the anaerobic digestion of the wastewaters were carried out using the kinetic parameters obtained from the batch digestion. The hydraulic retention time (HRT) and solid retention time (SRT) were mainly estimated by varying different model parameters like efficiency of reactor, influent substrate concentration and biomass concentration. The HRT was calculated for different reactor efficiency (E) varying in the range of (70-95%) at a constant influent substrate concentration. It was observed that the HRT increased with increase in efficiency for the three wastewaters with that of VOW ranging between 1.10 to 7.06 days. The values of SRT at a constant influent substrate concentration were observed to increase as efficiency increased with SDW having the highest value by ranging between 68.02 to 1,645 days. The effect of different substrate concentration on HRT value to attain a targeted efficiency in the homogeneous continuous flow type reactor under steady state condition was also studied on the digestion of the three wastewaters. It was observed that the HRT linearly increased with increase in influent substrate concentration for a particular value of reactor efficiency and biomass concentration with SDW having the highest value and ranging between 3.44 and 16.21 days. HRT also increased with decrease in biomass concentration and increase in reactor efficiency for a particular substrate concentration. SRT calculated at varying different influent substrate concentration at E1 (70%) and E2 (80%) efficiencies were observed to decrease with increase in influent substrate concentration. It was observed that SDW had the highest SRT by decreasing in the range of 95.14 to 43.25 days at 70% efficiency and 418.58 to 73.45 days at 80% efficiency. These values obtained could be used readily for reactor design.

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