No abstract

Based on worldwide works available in international literature, this paper describes the status of sewage sludge resulting from settleable solids in waste stabilisation ponds (WSP). This review presents, in detail, sludge characteristics, production and accumulation rates in order to provide background information to those who expect to advise or get involved with sewage disposal in situations where resources are limited. Knowing that several years are usually required for a sludge removal operation and that the long-term sustainability of WSP systems is dependent on the safe and effective management of their sludge, its cost must be estimated and taken into account in the annual maintenance costs of the processes. Thus, this paper intends to summarise desludging methods and their financial estimation. Even when ponds have been functioning for several years, most of the sediments are stabilised well, the final disposal is an issue in terms of risk due, for example, to their content in nematode eggs. More generally, the pathogen content in sludge from WSP ponds has to be known to define an appropriate management and to safeguard public health. Based on existing data, the rates and distribution of helminth eggs will be presented and practical treatment methods will be suggested. A number of sludge utilisation and disposal pathways will also be summarised. Sludge activity in terms of oxygen consumption is also discussed in order to gather more information to improve pond design and keep an economic and sustainable value of WSP. The objectives of the present review are to advance knowledge and gather scientific and technical information on all aspects of sludge management including production, characterisation, management, agricultural reuse and ultimate disposal.

The present study was undertaken to develop a simple and practical model for anaerobic digestion, encompassing sulphate reduction and sulphur oxidation, in a waste stabilization pond. The basic microbiological phases of the model consisted of four processes, namely acidogenesis, methanogenesis, sulphate reduction and sulphur oxidation. It also incorporated multiple reaction stoichiometry and substrate utilization kinetics. The study also aimed to investigate the mutual interaction between sulphate-reducing bacteria (SRB) and photosynthetic sulphur bacteria (PSB) in an anaerobic sludge consortia using batch reactors. The results revealed that for an initial concentration of sulphate ranging between 250 and 2800 mg x l(-1), SRB activity ranged between 20 and 190 mgSO4(2-)(reduced). The gVSS(-1) x d(-1) and PSB activity varied between 60 and 320 mgSO4(2-)(produced). gVSS(-1) x d(-1), and PSB activity was noted to be twice as high as that of SRB. PSB can, therefore, be used effectively in the fight against odors. The most important environmental factors affecting the sulphur cycle in the waste stabilization pond were likely to be the availability of sulphate and light for SRB and PSB, respectively.

Kinetic studies of nitrification and denitrification were carried out on reconstituted cores of sediments taken from wastewater stabilization ponds. This study aims to quantify the nitrification and denitrification in the sediment and to offer kinetic models to describe the processes. Sediment cores were collected, and laboratory studies were performed. The result showed that nitrification and denitrification processes are absent in the water column of stabilization ponds of the Bertrix (Belgium) wastewater treatment plant. On the contrary, nitrification and denitrification rates measured on 18 cores of sediment are, respectively, in the range of 0.12-1.56 g N-NH 4 + /m 2 day and 0.1-1.2 g N/m 2 day. In order to describe nitrification and denitrification processes, two kinetic models were developed using the Monod standard equation.