Hannah Rigby

Land application of sewage sludge (biosolids) in Australia: risks to the environment and food crops

Australia is a large exporter of agricultural products, with producers responsible for a range of quality assurance programs to ensure that food crops are free from various contaminants of detriment to human health. Large volumes of treated sewage sludge (biosolids), although low by world standards, are increasingly being recycled to land, primarily to replace plant nutrients and to improve soil properties; they are used in agriculture, forestry, and composted. The Australian National Biosolids Research Program (NBRP) has linked researchers to a collective goal to investigate nutrients and benchmark safe concentrations of metals nationally using a common methodology, with various other research programs conducted in a number of states specific to regional problems and priorities. The use of biosolids in Australia is strictly regulated by state guidelines, some of which are under review following recent research outcomes. Communication and research between the water industry, regulators and researchers specific to the regulation of biosolids is further enhanced by the Australian and New Zealand Biosolids Partnership (ANZBP). This paper summarises the major issues and constraints related to biosolids use in Australia using specific case examples from Western Australia, a member of the Australian NBRP, and highlights several research projects conducted over the last decade to ensure that biosolids are used beneficially and safely in the environment. Attention is given to research relating to plant nutrient uptake, particularly nitrogen and phosphorus (including that of reduced phosphorus uptake in alum sludge-amended soil); the risk of heavy metal uptake by plants, specifically cadmium, copper and zinc; the risk of pathogen contamination in soil and grain products; change to soil pH (particularly following lime-amended biosolids); and the monitoring of faecal contamination by biosolids in waterbodies using DNA techniques. Examples of products that are currently produced in Western Australia from sewage sludge include mesophilic anaerobically digested and dewatered biosolids cake, lime-amended biosolids, alum sludge and compost.

A critical review of nitrogen mineralization in biosolids-amended soil, the associated fertilizer value for crop production and potential for emissions to the environment.

International controls for biosolids application to agricultural land ensure the protection of human health and the environment, that it is performed in accordance with good agricultural practice and that nitrogen (N) inputs do not exceed crop requirements. Data from the scientific literature on the total, mineral and mineralizable N contents of biosolids applied to agricultural land under a wide range of climatic and experimental conditions were collated. The mean concentrations of total N (TN) in the dry solids (DS) of different biosolids types ranged from 1.5% (air-dried lime-treated (LT) biosolids) to 7.5% (liquid mesophilic anaerobic digestion (LMAD) biosolids). The overall mean values of mineralizable N, as a proportion of the organic N content, were 47% for aerobic digestion (AeD) biosolids, 40% for thermally dried (TD) biosolids, 34% for LT biosolids, 30% for mesophilic anaerobic digestion (MAD) biosolids, and 7% for composted (Com) biosolids. Biosolids air-dried or stored for extended periods had smaller total and mineralizable N values compared to mechanically dewatered types. For example, for biosolids treated by MAD, the mean TN (% DS) and mineralizable N (% organic N) contents of air-dried materials were 3% and 20%, respectively, compared to 5% and 30% with mechanical dewatering. Thus, mineralizable N declined with the extent of biological stabilization during sewage sludge treatment; nevertheless, overall plant available N (PAN=readily available inorganic N plus mineralizable N) was broadly consistent across several major biosolids categories within climatic regions. However, mineralizable N often varied significantly between climatic regions for similar biosolids types, influencing the overall PAN. This may be partly attributed to the increased rate, and also the greater extent of soil microbial mineralization of more stable, residual organic N fractions in biosolids applied to soil in warmer climatic zones, which also raised the overall PAN, compared to cooler temperate areas. It is also probably influenced by differences in upstream wastewater treatment processes that affect the balance of primary and secondary, biological sludges in the final combined sludge output from wastewater treatment, as well as the relative effectiveness of sludge stabilization treatments at specific sites. Better characterization of biosolids used in N release and mineralization investigations is therefore necessary to improve comparison of system conditions. Furthermore, the review suggested that some international fertilizer recommendations may underestimate mineralizable N in biosolids, and the N fertilizer value. Consequently, greater inputs of supplementary mineral fertilizer N may be supplied than are required for crop production, potentially increasing the risk of fertilizer N emissions to the environment. Thus greater economic and environmental savings in mineral N fertilizer application are potentially possible than are currently realized from biosolids recycling programmes.

Nitrogen availability and indirect measurements of greenhouse gas emissions from aerobic and anaerobic biowaste digestates applied to agricultural soils

Recycling biowaste digestates on agricultural land diverts biodegradable waste from landfill disposal and represents a sustainable source of nutrients and organic matter (OM) to improve soil for crop production. However, the dynamics of nitrogen (N) release from these organic N sources must be determined to optimise their fertiliser value and management. This laboratory incubation experiment examined the effects of digestate type (aerobic and anaerobic), waste type (industrial, agricultural and municipal solid waste or sewage sludge) and soil type (sandy loam, sandy silt loam and silty clay) on N availability in digestate-amended soils and also quantified the extent and significance of the immobilisation of N within the soil microbial biomass, as a possible regulatory mechanism of N release. The digestate types examined included: dewatered, anaerobically digested biosolids (DMAD); dewatered, anaerobic mesophilic digestate from the organic fraction of municipal solid waste (DMADMSW); liquid, anaerobic co-digestate of food and animal slurry (LcoMAD) and liquid, thermophilic aerobic digestate of food waste (LTAD). Ammonium chloride (NH4Cl) was included as a reference treatment for mineral N. After 48 days, the final, maximum net recoveries of mineral N relative to the total N (TN) addition in the different digestates and unamended control treatments were in the decreasing order: LcoMAD, 68%; LTAD, 37%, DMAD, 20%; and DMADMSW, 11%. A transient increase in microbial biomass N (MBN) was observed with LTAD application, indicating greater microbial activity in amended soil and reflecting the lower stability of this OM source, compared to the other, anaerobic digestate types, which showed no consistent effects on MBN compared to the control. Thus, the overall net release of digestate N in different soil types was not regulated by N transfer into the soil microbial biomass, but was determined primarily by digestate properties and the capacity of the soil type to process and turnover digestate N. In contrast to the sandy soil types, where nitrate (NO3-) concentrations increased during incubation, there was an absence of NO3- accumulation in the silty clay soil amended with LTAD and DMADMSW. This provided indirect evidence for denitrification activity and the gaseous loss of N, and the associated increased risk of greenhouse gas emissions under certain conditions of labile C supply and/or digestate physical structure in fine-textured soil types. The significance and influence of the interaction between soil type and digestate stability and physical properties on denitrification processes in digestate-amended soils require urgent investigation to ensure management practices are appropriate to minimise greenhouse gas emissions from land applied biowastes.

The use of alum sludge to improve cereal production on a nutrient-deficient soil

Alum sludge from wastewater treatment was applied at five rates on a phosphorus-deficient sand, and the effects on cereal growth and nutrition were investigated over 2 years. An inorganic fertilizer treatment, reapplied in the second year, was also included. The grain yield for inorganic fertilizer was 44% higher than the control in year 1 and 58% higher in year 2. Alum sludge was an adequate source of nitrogen for crop growth, and supplied sufficient residual nitrogen to meet crop requirements in year 2. However, grain yield in the alum sludge treatment applied at an equivalent available nitrogen rate to the inorganic fertilizer was 62% (year 1) and 69% (year 2) of the yield achieved by the inorganic fertilizer, though greater than the control. No toxic forms of aluminium were detected in the soil at any rate of alum sludge application. Plant shoot tissue analysis indicated that wheat sown in alum sludge-amended soil and the control were phosphorus deficient, whereas phosphorus was adequate in the inorganic fertilizer treatment. There was no evidence of any other nutrient deficiency. Alum sludge amendment resulted in an increase in soil phosphorus; however, further soil analysis indicated that forms of phosphorus present in alum sludge-amended soil may not be available for crop uptake; this is consistent with phosphorus deficiency observed in plant tissue in alum sludge-treated soil. It is suggested that on this nutrient-poor sand, the ability of alum sludge to provide sufficient phosphorus for plant production was limited in the 2 years after application.

Verification of an alternative sludge treatment process for pathogen reduction at two wastewater treatment plants in Victoria, Australia

At South East Water wastewater treatment plants (WwTPs) in Victoria, Australia, biosolids are stockpiled for three years in compliance with the State guidelines to achieve the highest pathogen reduction grade (T1), suitable for unrestricted use in agriculture and landscaping. However, extended stockpiling is costly, may increase odour nuisance and greenhouse gas emissions, and reduces the fertiliser value of the biosolids. A verification programme of sampling and analysis for enteric pathogens was conducted at two WwTPs where sludge is treated by aerobic and anaerobic digestion, air drying (in drying pans or solar drying sheds) and stockpiling, to enumerate and, if present, monitor the decay of a range of enteric pathogens and parasites. The sludge treatment processes at both WwTPs achieved T1 grade biosolids with respect to prescribed pathogenic bacterial numbers (<1 Salmonella spp. 50 g-1 dry solids (DS) and <100 Escherichia coli g-1 DS) and >3 log10 enteric virus reduction after a storage period of one year. No Ascaris eggs were detected in the influent to the WwTPs, confirming previous studies that the presence of helminth infections in Victoria is extremely low and that Ascaris is not applicable as a control criterion for the microbiological quality of biosolids in the region.