More efficient plant utilisation of the P in sewage sludge is required because rock phosphate is a limited resource. To meet environmental legislation thresholds for phosphorus (P) removal from wastewater (WW), primary treatment with iron (Fe) or aluminium (Al) coagulants is effective. There is also a growing trend for WW treatment plants (WWTPs) to be coupled to a biogas process, in order to co-generate energy. The sludge produced, when stabilised, is used as a soil amendment in many countries. This study examined the effects of anaerobic digestion (AD), with or without liming as a post-treatment, on P release from Fe- and Al-precipitated sludges originating from primary WWTPs. Plant uptake of P from Fe- and Al-precipitated sludge after lime treatment but without AD was also compared. Chemical characterisation with sequential extraction of P and a greenhouse experiment with barley (Hordeum vulgare) were performed to assess the treatment effects on plant-available P. Liming increased the P-labile fraction in all cases. Plant P uptake increased from 18.5 mg pot−1 to 53 mg P pot−1 with liming of Fe-precipitated sludge and to 35 mg P pot−1 with liming of the digestate, while it increased from 18.7 mg pot−1 to 39 and 29 mg P pot−1 for the Al-precipitated substrate and digestate, respectively. Thus, liming of untreated Fe-precipitated sludge and its digestate resulted in higher P uptake than liming its Al-precipitated counterparts. AD alone had a negative impact on P mobility for both sludges.
- anaerobic digestion
- lime treatment
- phosphorus recycling
- sludge stabilisation
- First received 12 August 2016.
- Accepted in revised form 16 January 2017.
- © IWA Publishing 2017