The Waste Strategy for England 2007 emphasises the importance of maintaining appropriate routes for recycling and disposal of sewage sludge to ensure that environment and public health are not put at risk. Agricultural recycling is considered the Best Practicable Environmental Option (BPEO). However, sludge cake does not have a universal appeal to the farming community due its inherent agronomic issues together with some logistical problems and environmental concerns. In general, phosphates in biosolids are more concentrated than nitrogen and traditional approach of applying the biosolids according to the nitrogen content may lead to accumulation of phosphates in the soil. Further, less predictable nutrients release characteristics of biosolids may lead to under or over fertilise the crop, hence, nutrient management following biosolids application could be challenging.
United Utilities and Cranfield University through a Knowledge Transfer Partnership have developed a biosolids derived novel product range for the agricultural market known as SMART-P™ Organo Mineral Fertilisers (OMF). These products are designed to overcome some of the identified agronomic limitations associated with current biosolids recycling practices. SMART-P™ OMFs are being used on farm scale crop trials to investigate agronomic performance when applied according to “good agricultural practices”. Further, these crop trials are also aiming to demonstrate the yield potential, possible phosphorus and heavy metal accumulation in comparison with conventional fertilisers.
Wheat crop responded to conventional fertiliser treatments quicker than for SMART-P™ OMF, nevertheless, no difference could be observed later in the season (eg: striping). However, wheat crop yield was significantly lower in SMART-P™ OMF treated plots compared with conventional fertilisers. Further investigation revealed that SMART-P™ OMF application based on total nitrogen in the product has under-fertilised the wheat crop by 22% “available nitrogen” compared to conventional fertilisers. However, forage maize and oilseed rape treated with SMART-P™ OMF have produced statistically similar crop yields compared to conventional fertilisers. At the end of the establishment year no detectable phosphate or heavy metal (Cu, Zn) build-up could be observed on any plots. However, long-term work is required to make firm conclusions on nutrient and heavy-metal build-up in continues SMART-P™ OMF application.
Keywords: Agricultural recycling; farm-scale crop trials; sludge management; Organo-Mineral Fertilisers; SMART-P™ OMF
Sludge is an unavoidable by-product of the sewage treatment process. Production is inevitable and cannot be curtailed (DEFRA, 2007). The Waste Strategy for England 2007 emphasises the importance of maintaining appropriate routes for recycling and disposal of sewage sludge to ensure the environment and public health are not put at risk (DEFRA, 2007a). Currently, the UK produces approximately 1.3 million tonnes (dry solid basis) of sewage sludge per annum and 62% of the total production goes to agricultural recycling after suitable treatments while 19% of the total production is disposed of through incineration with the remainder is mainly disposed to landfill and land reclamation (Water UK, 2005).
Application of biosolids to agricultural land is safe & sustainable and recognised as the Best Practicable Environmental Option (BPEO) in most circumstances by the EU and UK Government (Water UK, 2006). Application of biosolids provides significant amounts of organic matter to the soil which improves the soil structure, soil workability and water holding capacity (EA, 2004). Further, biosolids contain some valuable plant nutrients including nitrogen, phosphorus, sulphur and micro nutrients which could potentially replace costly mineral fertiliser inputs to the agricultural production system (DEFRA 2010). The Agriculture route for biosolids management also supports beneficial nutrient recycling from society to land, contributing to closing the loop of important nitrogen and phosphorus cycles (Lloyd, 2007).
As well as vital nutrients, sewage sludge contains heavy metals, pathogenic organisms and some toxic organic compounds (MAFF, 1993) hence, biosolids applications to agricultural land must be in accordance with the Sludge (Use in Agriculture) Regulations 1989 and the Code of Practice for the Agricultural Use of Sewage Sludge, to ensure that all applications are auditable and fully traceable (Water UK, 2006). In addition, Nitrate Vulnerable Zones (NVZ) Regulation also control biosolids applications based on its nitrogen input to the soil. The use of biosolids is also associated with some agronomic limitations due to its nutrient composition, nutrient release characteristics and current rates of application.
Nutrient composition of biosolids could vary based on many factors (effluent inflow, sludge treatment process etc.), but in general, phosphates are more concentrated in biosolids than nitrogen (Table 1). As a result, in practice, biosolids applications tend to supply substantially higher amounts of phosphates than many of the crops grown in the UK could utilise during a growing season (Table 2). Such repeated applications unavoidably may lead to an uncontrollable phosphate build-up in the soil. Phosphates are generally less mobile in the soil (Havlin et al, 1999), however, build-up of phosphates (above P index 4) could even lead to leaching losses in some soils (Bockman et al, 1990).