Removal of 95% biochemical oxygen demand (BOD), 85% COD, 90% ammonia, >65% total nitrogen, and >99% of the pathogenic indicator microorganisms from municipal wastewater can be achieved in algal ponds [469]. The effluent quality is highly variable: BOD = 10-25 mg/L and COD = 50-85 mg/L [470]. Oswald and col­leagues developed and tested the Advanced Integrated Wastewater Pond System (AIWPS) [471-475]. This system consists of methane production in an advanced facultative pond, algal high rate pond, algal settling pond, and maturation pond.

This system provides a secondary effluent adequate for agricultural irrigation and has 4-5 times lower electrical power consumption per water flow compared to con­ventional activated sludge and extended aeration systems [476]. A conventional wastewater treatment system requires approximately one kWhr of electricity for aeration for the removal of 1 kg of BOD. In contrast, photosynthetic BOD oxidation does not require aeration but produces algal biomass that can be converted to roughly one kWh of electricity through ADP [477] .

Algal Cultivation in Anaerobic Digester Effluent

Recent developments allow ADP to be applied for the treatment of a wide range of wastewaters with organic contamination. But ADP has several drawbacks particu­larly high organic matter and ammonium concentrations in the AD effluent. Moreover, AD has low efficiency of phosphorus removal. A similar waste liquid is generated during AD of algae. This nutrient rich anaerobic effluent can serve as fertilizer for intensive algal production.

Green microalgae (Chlorella and Scenedesmus) cultivated in diluted dairy waste anaerobic digester effluent are able to switch from phototrophic to heterotrophic or mixotrophic growth, utilize native substrates present in effluent, and increase the biomass and triglyceride production rate [478]. Ammonia removal efficiency from anaerobic treated dairy wastewater reached 96% with a mixed green algae culture [479] and 99% with A. platensis [480]. Aragon reported the removal of 85% BOD, 75% COD, 80% ammonia, and >97% detergents during treatment of the anaerobic effluent from an urban wastewater treatment plant by two local algae species Scenedesmus acutus and C. vulgaris [481].

A “closed” system of methane generation from light energy via algal production and anaerobic digestion was described by Golueke and Oswald [109]. The liquid phase from the digester was used as culture media for algal growth. The average methane yield was 0.44 L/gVS, the maximum energy conversion efficiency from light to biomass was 3%, and the energy conversion efficiency for the entire unit was 2%. Ras and colleagues repeated the same experiment with C. vulgaris as the solar light capturing organism [482]. The methane yield was 0.24 L/gVS at an HRT of 28 days and an OLR of 0.7 gVS/L-day.

Ryther compared the productivity of G. tikvahiae and Ulva sp. in media enriched by AD effluent with the productivity in a conventional mineral enrichment medium [483]. Ulva sp. had a similar methane yield in both media, but G. tikvahiae had 50-75% lower productivity in an AD effluent-enriched medium compared to a control.