A technological and economic exploration of phosphate recovery from centralised sewage treatment in a transitioning economy context

Abstract

Phosphate is one of the substances which wastewater treatment works (WWTW) have to lower in order to meet the South African regulatory discharge standard of 1 mg/L. Wastewater is increasingly viewed as a ‘water-carried waste’, presenting opportunities for resource recovery. South Africa has commenced its transition to a low-carbon and resource-efficient economy, all whilst it struggles to provide universal access to basic needs and is faced with massive infrastructure maintenance as well as upgrading backlogs in the sanitation sector in particular. Although phosphate recovery methods exist, there is little evidence to indicate that these techniques would be economically viable or socially accepted in South Africa. This paper explores the potential for centralized recovery of nutrients, through the conceptual design and techno-economic pre-feasibility assessment of two phosphate recovery options, at the largest WWTW in the Western Cape, South Africa. This assessment revealed that the digestate stream at the 200 ML/d Cape Flats WWTW (CFWWTW) has the potential to produce ~470 kg/d of struvite fertilizer, whilst recovering 4–8% of the plant’s costs in 20 years. When contrasted with the more familiar, yet less sustainable, chemical precipitation process, low-grade and high-grade struvite production establishment costs are 10 and 25 times higher, respectively. Still, to reduce effluent phosphate loading to within regulated standards, the low-grade struvite production option at an estimated net present cost of R25.4 million over a 20-year lifetime is more affordable than chemical precipitation at a net present cost of R51.2 million. Low-grade struvite production is thus concluded to be technically feasible and the economically most affordable option from a lifecycle-costs perspective. Although it is a simple process, it is not cheap. Municipalities will need to consider the lower operating costs, as well as the environmental benefit of producing a useful phosphate fertilizer, over the immediate capital investment, if they decide to install such an operation.

Keywords: nutrient recovery; struvite; techno-economic assessment; phosphate recovery; South Africa