Performance of the Phase II Dandora waste stabilisation ponds the largest in Africa: the case for anaerobic ponds

Abstract

This paper describes the performance of the Dandora Waste Stabilisation Pond System which treats the industrial and domestic sewage from the City of Nairobi, Kenya and is the largest pond system in Africa. The ponds have the capacity to treat a dry weather flow of 80,000m3/d but currently treat only half this flow. Increased sewage strengths over and above the design value mean that although the hydraulic load is halved the organic surface load on the primary facultative ponds is only 37% below the design loading of 190kg BOD5/ha/d. The results of the ongoing monitoring programme show that effluent quality is good and meets WHO guidelines for unrestricted irrigation. Extrapolation of the existing results show that the system is performing to predictable efficiencies for the existing organic load and retention times (>90% BOD5 removal and >6 log reduction in FC) and will continue to do so as design flows and loads are reached. A pilot anaerobic pond study with what turns out to be full size anaerobics, showed BOD5 removal in excess of 80% in the single celled anaerobic ponds at design loadings of 240g/m3/d against a design prediction of 53% removal. Most importantly anaerobic pond loadings as low as 17-26% of the permissable volumetric loading maintained their efficiency without odour problems. This allows anaerobic ponds to be operated at flows well below design levels without risk of odour and without the need therefore to by-pass the anaerobics during commissioning and low initial flows. The benefits of land saving in the tropics by including anaerobic ponds in the system design is discussed as are the benefits of detailed monitoring programmes for large pond systems.