Subsurface Injection of Activated Sludge: Response of two Grass Species

Abstract

Excess activated sludge produced at SITEL, the integrated effluent treatment plant of Pólo Petroquímico do Sul (South Petrochemical Complex), Triunfo, Brazil, is disposed of by subsurface injection on pangola grass fields at average rates of 24 tons/year (dry basis). Until now no definite destination has been found for the hay, due to its poor commercial value. With the purpose of investigating the behavior of more commercially valuable species under sludge application, a growth-leaching pot experiment is being conducted at the Soils Department of UFRGS (Federal University of Rio Grande do Sul). Oats (Avenastrigosa) and rye grass (Loliummultiflorum) were the first species to be tested. Sludge application rates varied from 0 to 6 times the recommended operational values. The following topics were studied to date: (a) uptake of mineral nutrients N, P, K, Ca, Mg, Cu, Zn and S; (b) translocation of heavy metals Cd, Pb and Cr to shoots; (c) leaching of nutrients and metals. Initial results showed that: (i) Sludge applied at rates up to 24 t/ha (dry basis) did not cause any phytotoxic effect. However, plant macronutrients released by sludge into soil did not reach the quantities required for proper agronomic management of the crops. (ii) Nitrogen was the growth limiting nutrient and also phosphorus to a lesser extent. Potassium was supplied mainly by the soil. (iii)Some constituents that appeared in leachate from chemically-treated pots did not leach from sludge-treated pots, showing an environmental benefit from sludge over mineral amendments in that respect. (iv) Mineral nitrogen, mainly nitrate, was detected in leachate for about 45 days, decreasing with time. (v) Cu, Zn, Pb, Cd and Cr did not appear in plant tops and leachate from sludge-treated pots in worrysome amounts. The investigation proceeds with cultivation of pangola grass and millet.