A series of secondary wastewater treatment simulation experiments were perfomed in the laboratory to determine the effects of feed composition on the activated sludge and to recommend an optimum feed that would result in a non-toxic effluent to aquatic animals.
OECD Porous Pots and Horizon Ecology Bio-oxidation units were used as test systems with standard OECD Porous Pot feed or various modifications of this feed. The units were seeded from a publically owned wastewater treatment plant and were run using similar parameters for hydraulic and solids retention times, dissolved oxygen, pH, mixed liquor suspended solids, etc. The Bio-oxidation unit was hard to maintain; its use was terminated after sludge solids could not be maintained at an adequate level, whereas the Porous Pots were easy to maintain and produced healthy, growing sludge.
The test animals used to evaluate effluent toxiclty were fathead minnows and daphnids. Daphnids showed more sensitivity to peptone and beef extract in the synthetic feed than they did to urea, whereas minnows were more sensitive to the urea. The feed that resulted in non-toxic effluents to both species was one in which nutrient broth and ammonium sulfate were substituted for peptone, beef extract, and urea. This feed is easy to prepare, grows healthy sludge, and produces an effluent that is non-toxic to aquatic animals such as fathead minnows and daphnids.