Transformation of Sulphur in Advanced Integrated Wastewater Pond System® Treating Tannery Effluent

An outdoor experiment was conducted to study the fate and transformation of sulphur in tannery wastewater being treated by a novel pond-based wastewater treatment technology known as the Advanced Integrated Wastewater Pond System® or AIWPS®. The experiment used a research-scale AIWPS® consisting of a primary Advanced Facultative Pond (AFP) with submerged Fermentation Pit (FP), Secondary Facultative Pond (SFP) and Maturation Pond (MP) all arranged in series. Screening and settling were included as sequential pretreatments. The sulphate and sulphide concentrations of the raw tannery wastewater varied between 150 and 3,500 mg/l and 100 and 800 mg/l, respectively. In a three-phased experiment, the AIWPS® facility was subjected to increasing organic and sulphate loadings. With sulphate loading rates of 78-244 g/m 3 -d over the experiment phases, the AFP with the FP attained a total sulphur (SO 4 =-S + S = -S) removal of 67-79%. Dissimilar sulphate reduction in the submerged FP followed by photosynthetic sulphide oxidation into elemental sulphur in the solar illuminated upper water layers was the major sulphur removal mechanism in the AFP. The overall organics removal performance of the AFP was not adversely affected by increased organics and sulphate loading rates. With an increase of sulphate loading from 78 g/m 3 -d during phase 1 to 122 g/m 3 -d during phase 2 and a corresponding COD loading increase from 560 to 1480 g/m 3 -d, COD removal efficiency in the AFP dropped by only 4%. Some process instability with inhibition of methane fermentation in the AFP and domination of microalgae by photosynthetic sulphur bacteria in the SFP were observed during phase 3 when the AFP was subjected to extreme sulphate and COD loading rates of 244 and 1,860 g/m 3 -d, respectively. The extreme loading rates decreased the COD removal efficiency of the AFP by only 25%. Odour free operations and high organics and sulphur removal performances of the AFP were due to the proliferations of microbial morphotypes similar to genera: Thiocystis, Rhodobacter, Rhodopseudomonas and Rhodospirillum. A sulphur scum with purity of up to 35% sulphur was recovered from the AFP water surface. Gentle mixing in the upper water layers of the SFP remedied the unwanted domination of algae by pink sulphur bacteria.