Statistical analysis of the performance of a biological nutrient-removal plant

Thames Water operated a demonstration-scale biological nutrient-removal pla nt for two years to investigate the feasibility of retrofitting it into exi sting activated-sludge plants. The plant was operated with and without the addition of short-chain fatty acids to the anaerobic zone. The results from the trial showed that phosphorus removal was possible, but that performanc e was not consistently good. The data show that the key parameters which af fect the effluent soluble phosphorus concentration are (i) the soluble phos phorus and oxidised nitrogen concentrations at the end of the aeration lane , and (ii) the sludge-blanket depth. Further analysis of the data suggests that secondary release of phosphorus was marked when the concentration of s hort-chain fatty acids in the anaerobic zone was low, even if the sludge bl anket was shallow and when oxidised nitrogen was present in the return slud ge. In contrast, secondary release of phosphorus was limited when the conce ntration of short-chain fatty acids in the anaerobic zone was high.