Stabilisation of dairy wastewater using limited-aeration treatments in batch reactors

Abstract

This research was conducted to study the treatment of dairy wastewater using limited-aeration treatments. The following process parameters were examined during the treatment process: (1) oxidation–reduction status; (2) organic matter degradation; (3) kinetics of bio-stabilisation; and (4) loss of ammonia. Results showed that during the first 3–4 days of limited-aeration treatments at 0.034 and 0.067 l [air] l −1 [manure] min −1 , dissolved oxygen (DO) remained close to the detection limit but, based on oxidation–reduction potential (ORP) levels (−30 to 100 mV), the wastewater oxidation–reduction status remained anoxic; the environment remained aerobic after this phase. The 70% maximum removal of both chemical oxygen demand (COD) and total volatile solids (TVS) observed by day 8 of treatment at either aeration rate indicated that the two-fold decrease in the aeration rate did not alter the ultimate reduction of the organic strength. Linear regressions performed on the COD against the TVS data obtained during this treatment indicate excellent linear relationships between the two parameters manifested in significantly high correlation coefficients ranging between 0.96 and 0.97. Either of the two parameters can thus be used to monitor the stabilisation of the wastewater and to determine the kinetics of bio-stabilisation. The kinetic studies established bio-stabilisation constants of 0.168 and 0.144 day −1 at the aeration rates of 0.067 and 0.034 l [air] l −1 [manure] min −1 , respectively. Accordingly, 50% bio-stabilisation (also referred to as half-life decay period) was achieved in approximately 4 and 5 days of treatment, respectively; meaning that reducing the aeration rate by 50% increased the half-life decay period by 25%, while reducing the decay rate by only approximately 14%. Effectively therefore, it is more economically prudent to aerate at the lower than at the higher airflow rate. In addition, a significantly higher ammonia loss was unaccounted for at the higher aeration rate (34% net loss) than at the lower aeration rate (23% net loss).