Nutrient recycling by Chlorella vulgaris from septage effluent of the Bangkok City, Thailand

Abstract

Only a small amount of septage (1,200 m3/day) from the Bangkok Metropolitan Region (BMR), Thailand, undergoes secondary treatment at a central wastewater treatment plant. The Nongkhaem Nightsoil Treatment Plant, however, has its treating capacity only 600 m3/day. More often, the effluent from the plant fails to meet with the industrial effluent standards as well as building effluent standards set by the government, especially for total kjeldahl nitrogen and total phosphorus. Thus, recycling nutrients back from this effluent before discharging to the public drains by waste reclamation via algal culture is of interest to improve water resource assimilation capacity. This study aimed to investigate bio-engineering factors such as light intensity and hydraulic retention time which influence nutrient uptake and growth development of Chlorella vulgaris in treated septage from the Nongkhaem plant. In a series of batch cultures of Chlorella vulgaris starting from 100 mg/L of biomass density, there was a significant increase in algal growth (p<0.05) with increasing light intensity (3,000, 5,000, and 8,000 lux). The culture conducted under natural sunlight (2,500-9,000 lux), however, gave the maximum algal and chlorophyll production within four days (390 mg/L and 5.8 mg/L, respectively), whereas the removal percentages of total Kjeldahl nitrogen (TKN) and total phosphorus (TP) were approximately 62% and 55%, respectively. Conversion of TKN into algal cell protein was equivalent to 24% and the water remaining after algal harvested after 4 days met Thai government effluent quality standards. It could be concluded that Chlorella vulgaris has the potential to be incorporated into a program of septage wastewater treatment for nutrients (N and P) polishing since only a short period (4-7 days) is required for the algal development to reach its full growth. However, harvesting of algae is necessary and should be done before it starts to decompose and releases those nutrients as well as organic substances to the effluent again.