Effects of hydraulic loading and laundry detergent on the operation of aerobic package treatment systems

This study focused on three potential problems with the operation of aerobic package treatment systems: hydraulic retention time, laundry detergents, and hydraulic surges. To determine the effect of hydraulic retention time on system performance, six bench scale activated sludge systems were constructed. Wastewater from an actual residence was collected twice per week and fed to the small activated sludge systems. Two of the systems had a hydraulic retention time of 2 days, two had a hydraulic retention time of 1 day and two had a hydraulic retention time of 0.5 days. Effluent quality was stable and good with regard to chemical oxygen demand (COD) and ammonia (NH)-N) and seemed to be independent of hydraulic retention time. All of the systems performed well, despite considerable variability in influent strength. To study the effect of high concentrations of laundry detergents on the operation of package treatment systems, three of the six laboratory systems were fed high concentrations of detergent. Other than some residual COD from the detergent, no effect on system performance was observed. The final component of the study was the modification of an existing package treatment system to equalize flows from an automatic washing machine. After a month of operation the modified system produced a more constant effluent quality, than did the unmodified system. The field system, with and without modification, had a low mixed liquor suspended solids (MLSS) concentration (35 mg/L). This was probably the result of the long hydraulic retention time. As a result of the low MLSS the system, with and without modification, did not meet Virginia effluent requirements for BODs, TSS or D.O. or generally accepted levels of NH₃-N.