Treatment of a combined dairy and domestic wastewater in an up-flow anaerobic sludge blanket (UASB) reactor followed by activated sludge (AS system)

The feasibility of using an up-flow anaerobic sludge blanket (UASB) reactor followed by activated sludge (AS system) for the treatment of wastewater discharged from dairy factory was explored. The UASB reactor was operated at a hydraulic retention time (HRT) of 24 h and organic loading rates (OLRs) ranging from 1.9 to 4.4 kgCOD/m3.d. The average total chemical oxygen demand (COD total) and total biological oxygen demand (BOD5total) concentrations of the UASB reactor effluent were 1385 and 576 mg/l, corresponding to percentage removal of 69% and 79%, respectively. Total suspended solids (TSS) and volatile suspended solids (VSS) removal averaged above 72% and 75%, respectively. Residual phosphorous and oil and grease concentrations of the UASB reactor effluent were 8.2 and 44 mg/l, corresponding to percentage removal values of 63% and 83%, respectively. This good performance could be attributed to the relatively long sludge residence time (SRT =76 d) imposed to the reactor. Total and faecal coliform counts were reduced in the treated effluent by a value of 1.07 and 0.9 log10, respectively. The net sludge yield coefficient was found to be 0.2 g VSS per g CODtotal removed per day, corresponding to 20% of the total influent COD imposed to the UASB reactor. The volatile solids / total solids (VS/TS) ratio of 0.66 of excess sludge revealed its good quality. Preliminary batch experiments of the AS system treating UASB reactor effluent indicated first-order removal kinetics between total organic carbon (TOC) and contact time. The TOC removal reached 80%, resulting in only 47 mg/l in the final effluent at a HRT of 2.0 h. Accordingly, the AS system was operated at a HRT of 2.0 h. The system achieved a substantial reduction of CODtotal, BOD5 total, TSS and oil and grease resulting effluent quality with residual values of only 35.0, 7.0, 14.0 and 2.8 mg/l, respectively. The geometric mean of total and faecal coliform counts was reduced by a value of 1.28 and 1.64 log10, respectively. Based on these results, it is recommended to use of an integrated system consisting of a UASB reactor followed by the AS system for the treatment of a combined dairy and domestic wastewater to produce a good effluent quality complying with the standards for discharge into agricultural drains.

The results obtained in this investigation revealed that the UASB reactor is an efficient technique for pre-treatment of DDWW at an average wastewater temperature of 20EC and a HRT of 24 h. The reactor achieved percentage removal values of 69% for CODtotal, 79% for BOD5total and 72% for TSS. The regular intentional sludge discharge from the UASB reactor overcome washout out of biomass. Moreover, the wasted sludge from the UASB reactor was almost stabilized (VS/TS ratio = 0.66). The sludge yield coefficient was around 0.2 g VSS per g CODtotal removed per day, corresponding to 20% of the total influent COD. Considering these results we recommended to apply an UASB reactor for pretreatment of DDWW at a HRT of 24 h and OLR of 3.4 kgCODtotal/m3.d. The combination of a UASB and AS system represent a very promising option for the treatment of DDWW at a total HRT of 26 h (24 h for UASB and 2.0 h for the AS system). This combined system achieved an overall removal efficiency of 98.9% for CODtotal, 99.6% for BOD5 total and 98.9% for oil and grease. The effluent quality of the combined system satisfies standards required for discharge into agricultural drains. Acknowledgments The authors would like to express their gratitude to Prof. Dr. Fatma El-Gohary for valuable discussion and the manager of Arab Dairy factory for helping us. The first author would like to thank Tarek Abdel-Shafi, Amer El-keleny and the technical staff of the Water Pollution Control Department for technical support.