Environment Protection Authority, Bankstown, NSW, 2200, Australia

Transcription

1 Scientific registration n o : 113 Symposium n o : 25 Presentation : poster Land management of effluent nutrients for protection of surface and ground waters Gestion en zone rurale des éléments nutritifs provenant des eaux usées pour la protection des eaux de surface et souterraines HIGGINSON Ross, BISWAS Tapas, SHANNON Ian Environment Protection Authority, Bankstown, NSW, 2200, Australia Introduction Large-scale intensive rural industries and sewage treatment plants in rural New South Wales have attracted the interest of community and regulatory authorities because of the potential of their effluents to contaminate land and water resources. Concern for the environment has made it preferable to recycle these effluents for irrigated agriculture, as the discharge of such effluent to rivers or oceans is no longer acceptable. Land application of effluent is receiving considerable attention at present as an alternative to discharge. Over recent years, land application has been increasingly regulated to protect human health and the environment from various potentially harmful constituents, such as pathogens, metals, organic chemicals and nutrients. Considering the high level of concern about the potential for pollution of surface and ground waters from effluent irrigation schemes, there is little published information in NSW where monitoring of such schemes has been undertaken. In NSW, submission of an environmental impact statement (EIS) is mandatory before any new development including effluent reuse scheme can take place. Usually, licensing requirements under pollution control legislation are imposed on developments once the schemes are in operation. The absence of a mutually agreeable set of assessment tools on many occasions has caused problems between developers, environmental consultants and the EPA. To overcome this, the EPA has developed a user-friendly, interactive computer model (ERIM) that can be used by the designers of effluent re-use sites and the government agencies that assess such sites. Effluent generation in rural NSW In NSW, about 80 million ha of land, or about two-thirds of the land surface, is used for farming activities (NSWSoE, 1997). Large quantities of effluent are generated from agricultural production and processing industries such as feedlots, piggeries, abattoirs, dairy farms, tanneries, and wool scours. Most agricultural effluents contain valuable nutrients that can be recycled onto land in order to improve soil fertility and increase the 1

2 sustainability of farming systems. For example, there are approximately 750,000 pigs in the State that produce about 5,500 ML (megalitres) of effluent annually (PRDC, 1997) and this effluent contains enough nitrogen to fertilise about 400,000 ha of wheat or barley. According to the MRC (1996), the red meat industry releases about 15 million tonnes of polluted water annually, which contains 2,000 tonnes of nitrogen and 600 tonnes of phosphorus. An estimate of effluent volumes produced by a range of rural industries across the State is given in Table 1. Table 1: Estimated effluent volumes produced by rural industries and STPs in NSW Source Volumes of effluent in ML/year NSW Australia Feedlots 200 3,500 Piggeries 5,500 20,000 Abattoirs 15,000 62,000 STPs 618,500 - Dairy farms - 28,000 Tanneries - 2,000 Sewage effluent production in the State has increased since 1989 from 300,000 to 681,500 ML/annum, an increase of around 127% (NSWRWCC, 1997) and is likely to increase in the future. Only about 6 percent of this effluent generated annually is applied to land. The remaining production is disposed of to rivers, estuaries or oceans and, assuming there is no further treatment, dumps a potential of about 20 million tonnes of nitrogen and 6.4 million tonnes of phosphorus to the environment. Effluent Quality Considerations Organic matter Table 2 gives an indication of industry effluent quality compared with sewage and good quality river water. BOD 5 must be balanced with nutrients for heterotrophic bacterial action, and to achieve this, a ratio of BOD:N:P of about 20:5:1 is considered ideal (Bowmer and Laut, 1992). The BOD 5 and suspended solids of tertiary treated sewage effluent make it suitable for discharge to water bodies. Many rural effluents, however, are very high in BOD 5 and much more concentrated than sewage. Most farm animals produce large quantities of waste each day compared with humans; equivalent values of BOD 5 being human = 1.0, cow = 16.4 and pig = 3.0. From Table 2, it is apparent that effluents from rural industries would never be suitable for direct discharge to water bodies. Nutrients: Nitrogen and Phosphorus Effluents containing total nitrogen of 3-10 mg N/L and total phosphorus of 0.3 to 2 mg P/L is allowed to be discharged to sensitive river or estuarine waters. However, concentrations above 0.1 mg P/L are considered to be sufficient in some circumstances to trigger algal growth in freshwater systems (ANZECC, 1992). Table 2 shows that only tertiary treated sewage effluent can meet this criterion and all other effluent must be applied to land or subjected to further processing. Piggery, feedlot, dairy, wool scouring, tannery and starch factory effluents are all are high producers of nitrogen. Conversely, effluent from abattoirs and tanneries are low in phosphorus. For example, if 5 ML of feedlot effluent were irrigated on a 1ha re-use area, then about 4,500 kg N/ha/year and 750 kg P/ha/year would be applied. Even if 50% of this N is considered 2

3 to be lost via NH 3 volatilisation, then the remaining load would be sufficient to cause nitrate and phosphorus contamination of water bodies. Table 2. Composition of effluent from from rural industries and communities in NSW, Australia. Industry/ Concentration in mg/ L BOD 5 EC SAR # Source ds/m Total N Tot. P Na K Ca Mg - Feedlot Piggeries Dairy Abattoir Winery Scouring Tannery Starch Sewage River * # SAR (Sodium Adsorption Ratio) = Na/ [(Ca +Mg)]; concentrations in mmol/l. * Murrumbidgee River Salts and sodium Both salts and adsorption ratio (SAR) limit the use of effluents. The total dissolved solid (TDS) concentration, a measure of the salinity of effluent, is an important determinant of its suitability for irrigation. Except treated sewage effluent, the TDS values for all other effluents are many times larger than that of potable water (EC 0.5 ds/m). Sodium salts are particularly important because excessive sodium in irrigation effluent can cause soil dispersion. Soil permeability and aeration problems occur when the SAR of irrigated effluent exceeds 6. The SAR values of piggery, dairy and sewage effluent are usually between 8.9 and This effect can be particularly severe with an effluent that has a low concentration of electrolyte, such as sewage, where both soil dispersion and structural damage can occur. Even with a high SAR value, both piggery and dairy effluents can maintain soil structure because of their high EC until the salinity begins to limit crop production and cause deterioration of surface and groundwaters. Many effluents are very high in potassium, such as wool scouring and winery effluents, which have concentrations of K + up to 3,500 to 7,000 times higher than typical NSW river water. This excess amount of K + has a deleterious effect on both soils and crops. If K + on the exchange sites of clay minerals exceeds 30 percent, it can then behave like Na + and disperse the clays. Normally no pesticides, heavy metals or synthetic organic compounds are found in intensive rural industry effluents. In effluent from the processing industries, these chemical contaminants, if present at all, are often in negligible concentrations with the exception of Chromium-VI in tannery sludge (Gardner et.al., 1996). Nutrient management and monitoring of re-use areas Spray irrigation is by far the most popular method of irrigation used, the aim being to balance nutrient and water application rates with plant uptake. Guidelines for achieving 3

4 these aims are given by the NSW EPA in its environmental guidelines for the utilisation of treated effluent by irrigation (EPA, 1995). The management of effluent irrigation has more to do with managing the supply of nutrients than water. The management of the land, and how it is cropped or grazed, will dictate how much effluent can be applied from year to year. Nutrients The nutrient that has received the most environmental attention in Australia is phosphorus as it is often implicated as the limiting nutrient for blue green algal outbreaks in river systems. Phosphorus fixation and retention mechanisms in most soils are considered to result in a low risk of P leaching and erosion of soil particulates remains the major export pathway for P (White and Sharpley 1996). For example, the depth of P leaching was reported to be less than 0.025m after 2.5 years of application of treated sewage effluent at Flushing Meadows, near Wagga Wagga, NSW (Falkiner and Polhgase, 1996). P leaching, however, can be of considerable concern in very sandy soils where cumulative P loadings exceed the P sorption capacity. Export levels as small as 0.4 kg/ha/year were correlated with rampant algal growth in the Peel-Harvey estuary of Western Australia (Birch, 1982), a very sandy catchment. In a starch effluent irrigation scheme, Lawrie (1996) found that high amounts of phosphorus from effluent accumulated in the topsoil but the concentration of orthophosphate in the groundwater was still below 0.04 mg/l after 9 years of operation. Whilst the Colwell P concentrations reached 1000 mg/kg in two of the topsoils, the concentration at cm depth at both sites was much lower, ranging between 50 and 100 mg/kg. Bray P measurements down the soil profile in 1993 confirmed that available P was below 5 mg/kg. The P sorption capacity of the subsoil was therefore very high. Unlike nitrogen, downward percolation of effluent P was poor. Lawrie (1996), after ten years of applying starch effluent to crops and pasture on an alluvial plain, concluded that high rates of addition (over 1000 kg N and 200 kg P/irrigated ha) and low rates of nutrient removal resulted in a considerable accumulation of nitrogen and phosphorus in the topsoil. Nitrate levels, particularly in the top 7.5 cm of soil, rose significantly. Even though evidence of nitrate leaching in the profile first emerged after 8 years of operation, there was no sign of groundwater contamination. A sample of water collected in the soil profile only 80 cm from the surface contained 1.3 mg NO 3 -N/L; the soil concentration at that depth was 9.1 mg/kg, and 93.9 mg/kg in the top 10 cm. Salts and Sodium Salt (NaCl) is very common in Australian soils and a recent environmental report (NSWSoE, 1997) states that about 5 million ha or 6% of the State are potentially saline soils. Salt accumulation in effluent irrigated soil can occur and reduce crop yield due to osmotic effects. Lawrie (1996) reports that irrigation of starch effluent (EC of ds/m) at Nowra for ten years resulted in substantial accumulation of salt in the sub-soil (Figure 1). Similarly, Smith et al. (1996) found that irrigation for over five years at Wagga Wagga with sewage effluent (EC of ds/m) also contributed to a substantial build-up of salt at depth in the subsoil (Figure 1). Effluent induced salinity at Nowra, however, did not reach the point where it was affecting crop growth. The 4

5 absence of salt accumulation in the soil profile at Nowra was attributed to the presence of a shallow water table and high permeability, which allowed the subsoil to be regularly flushed of salt by rainfall and drainage. On the other hand, soil salinity at Wagga Wagga was sufficiently high to reduce transpiration in tree plantations of Eucalyptus grandis despite the fact that the Nowra effluent was four to five times saltier than at Wagga Wagga. Figure 1. Soil salinity vs Soil depth: Nowra site is treated with Starch effluent and Flushing Meadow is treated with sewage effluent. At the same site, the exchangeable sodium percentage (ESP) of the surface 60 cm increased from 2% to greater than 25% after five seasons of irrigation (Balks et al, 1996). A rise in the ESP favours clay dispersion, but in both the cases, the permeability was not affected. To substantiate his claim of favourable effects of effluent irrigation, Lawrie (1996) added that there was increased soil organic matter level that decreased bulk density, increased porosity and aggregate stability. However the high level of ESP coupled with low level soil EC in the winter may produce swelling and dispersion of clay reducing the permeability of soil while increasing the volume of runoff. To remain flocculated, the clay will require a threshold electrolyte concentration to be maintained in the soil solution. To maintain this electrolyte concentration, strategic soil and water management practices will be required. Irrigation of effluent may therefore become a juggling act between managing salinity and sodicity. Impact on groundwater Land application of effluent is excluded from catchments where the surface water is used for domestic water supply. However, no such consideration is afforded to areas where groundwater is used for domestic purposes. Nitrate is highly mobile and can very easily leach to groundwaters and may concentrate there, making the water unsafe for drinking. While reviewing effluent management in intensive rural industries, Bowmer 5

6 and Laut (1992) report that in rural New South Wales, elevated nitrate concentrations have been found in the groundwater near piggeries, poultry farms, feedlots, dairies and septic tanks. Effluent Re-use Irrigation Model (ERIM) The EPA has developed a user-friendly, interactive computer model (ERIM) that can be used by the designers of effluent re-use sites and the government agencies that assess these sites. The ERIM model is based on the water balance equation, rainfall + effluent applied = evapotranspiration + runoff + drainage. It requires historical rainfall and evaporation; the amount of nitrogen and phosphorus introduced, their yearly removal by growing plants; amount of applied organic matter; and the water holding capacity of the soil. The model uses this information to estimate trade off between the land area required for irrigation and wet weather storage based on volume and strength of effluent applied. For this trade off relationship the phosphorus, nitrogen and BOD 5 loadings are determined. Minimum irrigation areas for each of these parameters are calculated and the largest area is the minimum irrigation area needed for sustainability. Case study a development scenario. 1n late 1996, the EPA received an application from a piggery which intended to expand its operation from 700 sows to 1000 sows (10,000 pigs). The property was on an alluvial river flat, with heavy dark silty clays, in northern NSW. The location had an average rainfall and evaporation of 965 mm and 1563 mm respectively. The effluent generated (20 L/pig/day) was to be discharged into a lagoon for wet weather storage, which would be used to spray-irrigate 90 hectares of wheat/pasture. The neighbours expressed strong concern about odour nuisance, pond overflows into the nearby creek, and contamination of groundwater with nitrogen. The local aquifer was used for potable, stock and irrigation purposes. ERIM was used to simulate the reuse scheme. The effluent contained 410 mg/l of BOD 5, 308 mg/l of organic N, 120 mg/l of ammonium N, 12 mg/l of nitrate N and 30 mg/l of total P. A critical BOD 5 loading rate of 28 kg/ha/day was accepted. Organic N mineralisation rate and ammonia volatization rate was assumed as 50% for each. The P sorption capacity was 158 mg/kg for a sorption depth of 1 m with a bulk density of 1.3 tonnes/m 3. The model was run using a daily effluent supply of 0.2 ML. Neither phosphorus nor BOD was limiting in the solution region but a minimum land area of 88 ha was required for nitrogen utilisation. Using the remaining part of the relationship, a storage of 33 ML and a land area of 88 ha, or a storage of 30 ML and land area of 120 ha were feasible solutions. No further reduction in storage below 30 ML was possible, even with an increase in land area available above 120 ha. This is due to the fact that the storage requirements are driven by the rainfall pattern and a minimum size is required to last through the cold/wet winter periods. Conclusion Land treatment of effluent is currently receiving considerable attention in NSW. Effluents are a valuable source of nutrients but their land application provides a potential for contamination of surface and groundwaters. Proper planning and 6

7 management of land application processes, including agronomic considerations, can substantially reduce or prevent such contamination. Experience in NSW has demonstrated that irrigation with both high and low strength effluents can be feasible, given proper management, without causing deterioration of the land and waters. A simulation model (ERIM) developed by the EPA has proved to be an excellent planning, assessment and explanatory tool for the protection of soils and waters. Bibliography ANZECC (Australian and New Zealand Environment and Conservation Council) Australian Water Quality Guidelines for Fresh and Marine Waters. ANZECC, Melbourne. Balks, M. R., Bond, W. J., and Smith, C. J. (1996). Effects of sodium accumulation on soil physical properties under an effluent-irrigated plantation. New Zealand Land Treatment Collective Proceedings of Technical Session. 14: Birch, P. B. (1982). Phosphorus export from coastal plain drainage into the Peel Harvey estuary system of Western Australia. Aust. J. Mar. Freshwater Res. 33: Bowmer, K. and Laut, P.(1992). Wastewater management and resource recovery in intensive rural industries in Australia. Water Res. 26(2): EPA (Environment Protection Authority) Guidelines for the utilisation of treated effluent by irrigation. EPA, Chatswood, Australia Falkiner, R. A., and Polgase, P. J. (1996). Movement of soil phosphorus in an effluent-irrigated plantation. 1 st Int. Conf.- Contaminants and the Soil Environment Extended Abstracts. February, 1996, Adelaide. pp Gardner, T., Atzeni, M., McGahan, E., Vieritz, A. and Casey, K. (1996). MEDLI-a computer model to help resolve environmental conflict in intensive rural industries, In Symp. Resolving Environmental Conflict, Institute of Engineers Australia, Brisbane. Lawrie, R. (1996). Medium and long-term impacts following irrigation of high strength effluent at Nowra. pp In Watertech Conf., Australian Water & Wastewater Association Inc., PO Box 388, Artarmon, NSW 2064, Australia. Meat Research Corporation (MRC), Environmental overview of meat processing projects and issues. Meat Research Corporation, Sydney, Australia NSWRWCC (New South Wales Recycled Water Coordination Committee) Effluent use in NSW 1995/1996 survey report, Department of Land and Water Conservation, PO Box 3720, Parramatta, Australia NSW SoE (NSW State of the Environment) SoE Report-97. EPA, Chatswood, Australia PRDC (Pig Research and Development Corporation) PigStat-96, PRDC, PO Box 4804, Kingston, ACT, Australia Smith, C. J., Snow, V. O., Bond, W. J. and Falkiner, R.A. (1996). Salt Dynamics in effluent irrigated soil In Land Application of Wastes in Australia and New Zealand: Research and Practice (Eds. P J. Polglase and W. M. Tunningley) pp Proceedings 14 th Land Treatment Collective Meeting, Canberra, 30 September-1 October. White, R.E., and Sharpley, A.N. (1996). The fate of non-metal contaminants in the soil environment. In Contaminants and the Soil Environment in Australasia-Pacific Region (Eds R. Naidu, R.S. Kokana, D.P. Oliver, S. Rogers, and M.J. McLauglin) Kluwer, Dordrecht. Key words: effluent irrigation, environmental model, intensive rural industries, nutrient balance, soil pollution, water pollution. Mots clés : irrigation avec des eaux usées, modèle environnemental, industries agricoles intensives, balance en nutriments, pollution des sols, pollution des eaux 7