Runoff Water Contamination by Phosphorus from Sludge, Effect of Phosphorus Removal and Sludge Conditioning
Abstract:The aim of this study was to assess the consequences of the agricultural use of phosphorus (P) enriched biosolids, on no-point P losses to surface water.
The dissolved and particulate P (DP and PP) content of the runoff water was monitored on the field, after sludge application. Three simulated storm events as well as the natural rainfall events during a one-year period were studied. Thickened, dewatered and dry sludges were used. Three dewatered sludges were applied in order to assess the effect of lime addition, chemical P removal and enhanced biological P removal in the WWTP.
The only noticeable effect was observed during storm events occurring a week after liquid sludge application, which induced an increase of the DP concentration from 0.15 mg.l−1 to 0.57 mg.l−1. On the other hand, the liquid sludge preserved the soil surface from sealing and crusting. This contributed to decrease the runoff intensity and therefore the erosion rate: the load of sediment exported during the simulation was 290 m3 ha−1 on a reference surface and 130 m3 ha−1 on the surface supplied with sludge. The PP losses, that accounted for more than 95 % of the total losses, correlated with the erosion and decreased from 2.8 kg ha−1 to 1.4 kg ha−1.
The P losses due to natural rainfall events ranged from 50 to 200 g ha−1. They correlated with the erosion intensity but were not influenced by sludge application. Nevertheless, the lixiviation of P from the biosolids particles on the soil ranged from 7 to 16 g P per kg of dry sludge applied. The most intense lixiviation was observed on dewatered sludge produced by a WWTP including enhanced biological P removal. Lime or FeSO4 addition as well as drying contributed to reduce P lixiviation from the sludge.
This study concludes to low risk of P contamination of surface water by P-enriched sludges. It also gave evidence of processes and kinetics of P release from the sludge, in the field. This should be taken in account in fixing the maximum sludge load on soils, in area sensitive to eutrophication.
Document Type: Research Article
Publication date: January 1, 2000
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