In tropical regions, where most of the developing countries are located, septic tanks and other onsite sanitation systems are the predominant form of storage and pre-treatment of excreta and wastewater, generating septage and other types of sludges. The septage is disposed of untreated, mainly due to lack of affordable treatment options. This study presents lessons that have been learned from the operation of pilotscale constructed wetlands (CWs) for septage treatment since 1997. The experiments have been conducted by using three CW units planted with narrow-leave cattails (Typha augustifolia) and operating in a vertical-flow mode. Based on the experimental results, it can be suggested that the optimum solids loading rate be 250 kg TS/m2 yr and 6-day percolate impoundment. At these operational conditions, the removal efficiencies of CW units treating septage at the range of 80-96% for COD, TS and TKN were achieved. The biosolid accumulated on the CW units to a depth of 80 cm has never been removed during 7 years of operation, but bed permeability remained unimpaired. The biosolid contains viable helminth eggs below critical limit of sludge quality standards for agricultural use. Subject to local conditions, the suggested operational criteria should be reassessed at the full-scale implementation.

An overview of the current literature-based knowledge regarding faecal sludge (FS) treatment along with the results and conclusions from own field research are presented. Issues for further faecal sludge treatment studies are also addressed. The article focuses firstly on the characteristics of the various types of faecal sludge. A set of variables is proposed for minimum faecal sludge determination and faecal sludge treatment plant (FSTP) design and control. Priority treatment options particularly relevant to developing and newly industrialising countries are listed. They include solids-liquid separation by settling/thickening processes, sludge dewatering and drying in drying lagoons or drying beds, stabilisation ponds, and co-composting with refuse or other bulking/organic material of high carbon content. The results of in-depth monitoring of a faecal sludge treatment plant in Accra, Ghana, are reported. The plant receives septage and public toilet sludge and comprises solids-liquid separation by settling/thickening followed by a series of four ponds for the treatment of the liquid fraction. The four ponds in series all function as anaerobic ponds. Facultative pond conditions do not develop. This appears to be due to the suppression of algal growth through high levels of ammonia (NH3). Final effluent BOD is 300 mg/l as against 2,000 mg/l in the raw sludge mixture and 1,600 mg/l in the sedimentation tank effluent.

Constructed wetlands (CWs) have been proven to be an effective low-cost treatment system, which utilizes the interactions of emergent plants and microorganisms in the removal of pollutants. CWs for wastewater treatment are ndrmally designed and operated in horizontal-flow patterns, namely, free-water surface or subsurface flow, while a vertical-flow operation is normally used to treat sludge or septage having high solid contents. In this study, three pilot-scale CW beds, each with a surface area of 25 m2, having 65 cm sand-gravel substrata, supported by ventilated-drainage system and planting with narrow-leave cattails (Typha augustifolia), were fed with septage collected from Bangkok city, Thailand. To operate in a vertical-flow mode, the septage was uniformly distributed on the surface of the CW units. During the first year of operation, the CWs were operated at the solid loading rates (SLR) and application frequencies of, respectively, 80-500 kg total solid (TS)/m2 x yr and 1-2 times weekly. It was found that the SLR of 250 kg TS/m2 x yr resulted in the highest TS, total chemical oxygen demand (TCOD) and total Kjeldahl nitrogen (TKN) removal of 80, 96 and 92%, respectively. The TS contents of the dewatered septage on the CW beds were increased from 1-2% to 30-60% within an operation cycle. Because of the vertical-flow mode of operation and with the effectiveness of the ventilation pipes, there were high degrees of nitrification occurring in the CW beds. The nitrate (NO3) contents in the CW percolate were 180-250 mg/L, while the raw septage had NO3 contents less than 10 mg/L. Due to rapid flow-through of the percolates, there was little liquid retained in the CW beds, causing the cattail plants to wilt, especially during the dry season. To reduce the wilting effects, the operating strategies in the second year were modified by ponding the percolate in the CW beds for periods of 2 and 6 days prior to discharge. This operating strategy was found beneficial not only for mitigating plant wilting, but also for increasing N removal through enhanced denitrification activities in the CW beds. During these 2 year operations, the dewatered septage was not removed from the CW beds and no adverse effects on the septage dewatering efficiency were observed.

Waste stabilisation ponds are a widely accepted and proven technology to treat wastewater. It is often stipulated (and also practised) that the design of ponds for faecal sludges (FS), septage in particular, should follow the same principles as for wastewater. Field research conducted by SANDEC and its partners at the Water Research Institute in Ghana, and information gathered from the scarce literature on FS treatment, however, has shown that design principles should be taken into consideration, which are specific to the treatment of FS. These principles depend on the type of FS to be treated and on the type of pond system envisaged. Issues dealt with in this article are the handling of FS solids; the role of anaerobic ponds in FS treatment; the anaerobic degradability of septage; and ammonia (NH3–N) toxicity.

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