Aquacultural approaches to recycling of dissolved nutrients in secondarily treated domestic wastewaters—I Nutrient uptake and release by artificial food chains

Abstract

Wastewater based artificial food chains composed of sequential monocultures of a suspended unicellular green alga, of an herbivorous cladoceran crustacean, and of an herbivorous or a carnivorous teleost fish and of filamentous green algae were studied in an effort to find efficient, potentially economically viable mechanisms for the biological capture (tertiary treatment) of nutrients (especially nitrate and phosphate) in wastewaters processed by small to medium size two-stage treatment plants that primarily handle domestic and agricultural wastes. All experiments were carried out on laboratory scale systems with a low technology approach to keep apparatus and procedures as simple and reliable as possible. Among all organisms tested as components of the artificial food chains only the unicellular green alga Scenedesmus, the filamentous green algae Cladophora and Ulothrix, the cladoceran crustaceans Daphnia magna and Daphnia pulex, and the teleost fishes Notemigonus crysoleucas, Pimephales promelas and Notropis lutrensis were found to be suitable. Unmodified secondarily treated domestic effluents were both iron deficient and lacking in buffering capacity necessary for optimal algal growth. Additions of both ferrous iron (1 ppm) and carbon dioxide (5%) were required to achieve good quality cultures suitable for feeding cladocerans. Most removal of nutrients occurred in the first algal stage of the food chain. Nitrate removal averaged 78%, phosphate removal 55% in buffered algal cultures. In unbuffered cultures nitrate removal was 30%, phosphate removal 98%. The near complete removal of phosphate in unbuffered algal cultures was probably due to physico-chemical precipitation of phosphate complexes formed as a result of high pH levels (pH > 10) reached within 24 h of culture initiation. Cladoceran and fish stages added nutrients (mostly ammonia and phosphate) back into the effluent. A final stage of Ulothrix and Cladophora algae removed nutrients regenerated by cladocerans and fishes. A 12 h light–12 h dark cycle statistically significantly reduced levels of nutrient removal by both unbuffered and buffered algal cultures below removal rates measured in algae cultured in continuous light. Daily harvesting rates of 25–75% of culture volumes had no significant effect upon removals of nitrate by buffered algal cultures; phosphate removals were inversely proportional to harvesting rates in these cultures. Important theoretical and technical points are discussed.