Phosphate in sewage discharges is a major cause of eutrophication in freshwaters. EC legislation requires stripping of phosphate from sewage effluents discharged to waters deemed sensitive to eutrophication, and it is likely that a growing number of sewage works will have to use the technology (ENDS Report 269, pp 13-14 ). The process produces phosphate-rich sludges which will present a disposal problem for the water industry.
However, firms which use phosphate in products such as detergents, flame retardants, paints and food additives regard these sludges as a potentially valuable resource. John Driver, chairman of the phosphate industry's common research fund (CEEP), says that several - including Hoechst, Rhone-Poulenc and Albright & Wilson - are collaborating in a £170,000 research project to explore the feasibility of recovering phosphorus from sewage.
World phosphate rock reserves are sufficient to last 200 years, but the quality is declining while extraction costs are rising. Processing of the rock produces wastes contaminated with cadmium and other toxic metals which may be incorporated into fertilisers - and hence added to soils - discharged to water or landfilled. Phosphates in sewage-derived sludges contain a factor of 10 or 100 times less metals, Mr Driver estimates.
The phosphorus content of sewage in the UK is about 45,000 tonnes per year - about 2g per person equivalent per day. Some 7,000 tonnes are recycled to agricultural land by sludge spreading, and only part of the remainder is likely to be recoverable. Nevertheless, sewage could supply a significant proportion of the 100,000 tonnes of phosphorus which the industry uses in chemical products each year.
A major barrier to phosphate recovery is that sewage treatment technologies which allow phosphorus to be recovered in a usable form have not been fully developed. To stimulate interest, CEEP has commissioned a study of phosphorus removal and recovery technologies.1 One of the most widely used methods - precipitation using iron or aluminium salts - produces a sludge which the industry finds impossible to use.
Another possibility is recovering phosphorus from sludge incinerator ash. However, ash from primary sludges may have a high iron content which would pose processing problems.
The most promising technologies are biological systems which produce sludges rich in phosphorus and low in metals. Fairly pure calcium phosphate could be made at sewage works from these sludges by precipitation with lime.
"The chemistry is simple but making it work on a large scale is not," says Mr Driver. The water industry would have to adopt an appropriate technology on a large scale to make the process viable. Phosphate processors would have to adapt to using the material, but its purity would enhance its value through reduced processing and waste disposal costs.
Per tonne of phosphorus, phosphate rock currently costs the industry about $300, to which processing costs add at least a further $230. Rough calculations suggest that relatively pure calcium phosphate from sewage might be worth $75 per tonne. Mr Driver contrasts this with the costs of disposing of phosphate-rich sludges in the USA which may be as high as $100 per tonne.