Phosphorus recovery as struvite from digested sludge - experience from the full scale.

There is an ongoing paradigm shift; wastewater is often not considered a waste any more, but a source of valuable resources: nutrients (N: nitrogen, P: phosphorus, and K: potassium), energy and water. The recovery of phosphorus from municipal wastewater has gained a lot of attention because of limited phosphate rock reserves and associated geopolitics, and pollution of phosphate rock. At the WWTP of Leuven, Aquafin operates a full scale installation to recover phosphorus as struvite from digested sludge. This paper discusses the performance of the struvite plant, pollutants in the struvite, struvite use, and economics.

Closing the phosphorus cycle: Multi-criteria techno-economic optimization of phosphorus extraction from wastewater treatment sludge ash.

The presence of heavy metals in concentrations above legal limit values is one of the main obstacles preventing closure of the phosphorus (P) cycle through directly applying wastewater treatment sludge ash as a fertilizer. Therefore, an alternative procedure is proposed to recover the valuable P from the sludge ash via wet chemical extraction. This comprehensive study uses several inorganic and organic acids, chelating agents and an alkaline solution to establish optimal and cost-effective conditions for wet P extraction from sludge ash. The optimization takes into account co-extraction of the following heavy metals: Cd, Cr, Cu, Ni, Pb and Zn. Design of experiments results show extraction liquid concentration, liquid/solid ratio and contact time all affect P and heavy metal extraction efficiency, both individually and through interaction. In addition, type of extraction liquid and pH at the end of the extraction procedure also affect P and heavy metal extraction efficiency. Combining results of XRD and SEM-EDX analysis with extraction data shows that at a pH <2, both Ca- and Al-phosphates in the ash dissolve easily. However, at slightly higher pH only Ca-phosphates dissolve well and at alkaline pH only Al-phosphates. The best trade-off between high P extraction, low heavy metal co-extraction and low operational costs is obtained with H2SO4 (0.5 N, 10 ml/g, 120 min) and oxalic acid (0.5 N, 12.8 ml/g, 120 min). H2SO4 outperforms the other extraction liquids in terms of extraction liquid costs per kg P extracted, whereas extraction with oxalic acid results in the lowest heavy metal co-extraction, thus reducing the downstream processing costs. None of the extraction liquids considered is appropriate for heavy metal removal prior to P extraction due to loss of P and insufficient heavy metal removal.

Full-scale phosphorus recovery from digested wastewater sludge in Belgium - part II: economic opportunities and risks.

One of the options to recycle phosphorus (P) in the wastewater sector is to recover it as struvite crystals from digested sludge. Measurements on a full-scale demonstration plant in Leuven, Belgium, yielded a first indication of the profitability of struvite recovery, in function of different variables such as incoming PO(4)(3-) concentration, MgCl2dosing, improved dewaterability, etc. An uncertainty and sensitivity analysis was carried out. Although possible improvement in sludge dewaterability when recovering struvite from digested sludge has a positive economic amortization effect, it is at the same time the largest source of financial risk. A theoretical exercise showed that for struvite recovery from centrate, uncertainty would be lower, and the largest sensitivity would be attributed to ingoing PO(4)(3-) concentration. Although struvite recovery from digested sludge is riskier, it is an investment with potentially a higher return than investment in struvite recovery from centrate. The article provides information for possible financial incentive schemes to support P-recovery.