WRc develops safe sludge strategy for Cairo
Principal consultant Jeremy Hall reports on a study of sludge disposal techniques for the city of Cairo, which has just been completed by the WRc.
Around 400,000t (dry wt) of sludge is produced at Cairo’s STWs every year. Agriculture was considered unsuitable due to the quantities involved and concerns over sludge quality. Up until the early 1990s it was thought that the only solution would be to dump the sludge in the desert.
However, depending on the quality of the sludge and the treatment processes available, the use of sludge as a soil conditioner and fertiliser is probably the best option from an environmental point of view.
This is particularly the case in arid countries such as Egypt where agricultural expansion onto desert soils is necessary to feed the growing population.
Farmers are prepared to pay for sludge as an alternative to farmyard manure, the latter being in short supply due to the decline in the use of draught animals. Also the cost of fertiliser has increased sharply in recent years, as government subsidies have been removed.
The Cairo sludge disposal study was funded in 1995 by the EC’s ‘third countries’ programme, the Mediterranean environmental technical assistance programme (METAP) and the European investment bank (EIB). It was promoted locally by the Cairo wastewater organisation (CWO).
The study was managed by WRc plc, with the assistance of local consultants. The main objective was to provide scientific and practical advice to sludge producers, environmental and health protection authorities and farmers, for the sustainable and safe use of sewage sludge in agriculture.
The study involved three phases over a period of 45 months. Phase one was a review and planning stage, involving a world-wide survey of information and the development of sampling and field trials carried out over three years, during phase two.
During phase three, a number of the key outputs were produced; scientific reports of the findings of phase three; extension and farmers’ guides to the practical use of sludge (in Arabic); and a management plan for the agricultural use of sludge for Cairo’s wastewater services.
Analysis of sludge quality was carried out in a three-year sampling programme at all of Cairo’s STWs.
The chemical and microbiological quality of liquid, dried raw and digested sludges were analysed. The results showed conclusively that the chemical quality of Cairo sludges is quite acceptable for agricultural use.
Heavy metal concentrations were similar to those found in Europe, and in the most industrialised catchments, concentrations rarely reached limit values (equivalent to US EPA ‘exceptional’ sludge quality).
In countries such as Egypt, the hygienic quality of sludge is the most quality-critical factor.
Enteric infections are prevalent in the population and this is reflected in a high occurrence of pathogens and parasites in raw sludge.
With low labour costs, sludge is normally spread on land by farm workers who are in close contact with the sludge. The priority is therefore to treat the sludge to a level which is safe for them. If this could be achieved, it would also provide a high level of protection for consumers of crops.
The challenge is how to achieve the required levels of disinfection reliably and at a reasonable cost under the local conditions.
A significant proportion of sludge in Cairo will be treated by anaerobic digestion in the future, but unfortunately this will not remove all parasite eggs, particularly those of the worm Ascaris lumbricoides.
The recommendation of the study was to improve the management of the traditional air-drying system to maximise desiccation and exposure to solar radiation, as a simple and robust method of achieving satisfactory quality, provided the sludge is stored for an extended period.
To gauge effects on crop growth, a series of 30 trials were established on six sites, covering 83ha. The trials were designed to evaluate sludge as a replacement for farm manure for a wide range of crops.
Fertiliser treatments were included in the trials so that the nutrient replacement values of the sludges could be derived statistically. Methods of irrigation and sludge application were also compared.
In Egypt, two arable crops are normally grown each year and so many trials had a total of six seasons of cropping, allowing the effects of crop rotation to be studied intensively.
The results showed conclusively that sludge improved the nutrient content of the crops, and that heavy metal uptake by the plants was negligible.