A User's Guide to Research on Application of Organic Wastes to Land
The principal objective of this report is to draw together the findings of research on wastes and waste recycling to land in an accessible format for the user-community as a User's Guide.
The wastes considered are agricultural wastes, industrial wastes exempted for landspreading and sewage sludge.
The Guide is intended to address the following issues:
- principles of recycling;
- human and animal health;
- contaminants (potentially toxic elements, organic pollutants);
- odour and aesthetics;
- agricultural benefit and ecological improvement;
- public perception and communication;
- operational aspects relevant to landspreading in Scotland and Northern Ireland.
It was intended that, as far as possible, each of the above sections would present information on the topic heading which was applicable to all three categories of waste (farm wastes, industrial wastes and sewage sludge) since the principles of beneficial recycling and environmental protection apply to all of them Properties and issues associated with particular wastes are considered where appropriate within each section
The main findings were as follows:
- It is estimated that about 22 million tonnes (dry weight basis) of waste is recycled to land each year in the UK Of this, 94% is waste from farm animals, 2% is sewage sludge and 4% is industrial waste of which half is paper industry waste.
- Approximately 15 million tonnes (fresh weight) of organic agricultural manures are applied to farmland in Scotland every year. Approximately 80% of this manure is in the liquid form with the remainder being solid. Most of the farm manure used in Scotland is derived from cattle excreta The fresh weight of exempt wastes annually applied to farmland in Scotland is a minimum of 250 000 tonnes yr-1 The main exempt wastes used in Scotland are paper mill sludges, distillery waste and milk processing waste. Only 0.21% of the total area of agricultural land in Scotland (excluding rough grazing) received sewage sludge in 1996/97 although this may increase with loss of the sea disposal outlet for sludge at the end of 1998.
- A total of 367 agricultural pollution events were recorded for 1997 in Scotland, which represents an 11% increase over the 1996 figure. This was the first rise in the number of substantial pollution events directly attributable to agricultural activity in three years. However, the percentage of major events, amounting to 16% of this total, appears to have stabilised. Incidents related to the spreading of animal waste on land have increased to their highest level since 1993, indicating poor planning in the utilisation of livestock slurries and manures. Applications of slurry in the winter months, where there is little or no nutrient requirement for crop growth and when prevailing weather conditions are considerably worse, are inherently risky from an environmental point of view (SAPG, 1998). SAPG (1998) concluded that there is a key role for detailed Farm Waste Management Plans (FWMPs) and improved field supervision as a means of minimising the risk of this type of pollution.
- Farmers in Northern Ireland use approximately 100 000 t of nutrient nitrogen (N) each year at a value of £40 million at current prices (Garrett, 1998). In addition, approximately 30 000 t of N is supplied to Northern Ireland (NI) farmland each year in the form of organic manures (Long and Gracey, 1990). Approximately 17 500 tds of sewage sludge was recycled to land in 1996/7 which required 0.14% of the available agricultural land, excluding rough grazing. A review of pollution from diffuse agricultural sources in Ireland was carried out by Tunney et al., (1998). Winter applications of slurry are necessary when sufficient slurry storage capacity is not available on farms. The Northern Ireland Farm Waste Survey (Tunney et al., 1998) found that slurry storage capacity was less than four months on 39% of farms, leading to the likelihood of slurry spreading in January-February for cattle housed from October. Only 25% of farms had seven or more months' capacity which would enable slurry storage over the entire winter. In terms of application times, this lack of capacity on farms is evident from the increase in slurry spreading recorded for cattle farms in January and February when 25% of slurry was applied.
- Landspreading of wastes must be justified on the basis that it will achieve agricultural benefit or ecological improvement. Whilst agricultural benefit from landspreading of wastes can readily be demonstrated there will be comparatively few instances where landspreading of wastes can be justified on the basis of ecological improvement. In this sense at least, agricultural benefit is more important than ecological improvement as a justification for landspreading of wastes.
- Agricultural benefit will be achieved when an application of a waste to land improves soil conditions for crop growth whilst ensuring the protection of environmental quality in the broadest sense.
- Ecological improvement is associated with the maintenance of habitats and their biodiversity where these would otherwise deteriorate, the provision of new habitats for wildlife and the development or restoration of existing habitats to give greater biodiversity and sustainability.
- The fertiliser replacement value of organic wastes should be taken into account when they are used on the land and applications of other fertilisers and manures reduced accordingly so that crop requirements for nutrients are not exceeded. The difficulty lies in accurately estimating the availability to crops of the nutrients in sewage sludges and industrial wastes even assuming that they are of consistent quality when supplied to farmers. Nevertheless, it is bad practice and potentially polluting to ignore their nutrient content and to continue to apply the full crop requirement as inorganic fertiliser in addition to the dressing of organic waste. Agricultural trials to define the fertiliser replacement value of organic wastes and a commitment by producers to supply a consistent product would pave the way for better management of organic wastes on the land.
- Mixing of wastes before landspreading is an option to be considered with caution and with the assistance of PQA (properly qualified advice). On a precautionary basis, there should be consultation with the regulatory authority before any landspreading operation based on mixtures of wastes proceeds.
- Wastes likely to contain pathogens include: Sewage sludge - treated and untreated - septic tank and cesspit waste (only septic tank sludge is exempted for landspreading), and animal manure and slurry.
- If wastes contain the agents of infectious diseases (pathogens), health hazards may arise to those applying the waste, farm workers and animals. There is also an indirect risk to the animal handlers, including market and abattoir workers; butchers and related trades handling contaminated meat; farm and market workers handling vegetables grown in contaminated ground, as well as the general public coming into contact with the waste or contaminated foodstuff (meat products, dairy products or vegetables).
- The risk of disease transmission diminishes rapidly the further the contact is removed, in the context of the infection chain, from the original application of a waste. Good hygiene practices at farms, abattoirs, markets, wholesale and retail outlets, and in kitchens will considerably lower any risk, which should already be low as a result of adherence to the appropriate codes of practice.
- In the context of the protection of the health of humans and other animals in contact with wastes during or after landspreading there are three principal routes of infection with pathogens; through the mouth, through wounds or through the respiratory system.
- The great majority of viruses are obligate parasites, i.e. unable to multiply outside the host's body, and highly specific in their potential host range. Thus, as a general rule, pig slurry would not present a virological hazard to cattle and vice versa, and neither would be hazardous to humans. Therefore the main health risk from viruses from the land application of animal wastes will be to other livestock. Because of the technical difficulty of isolating and identifying viruses there have been relatively few studies of viruses in slurries and wastes, and even fewer on the hazards presented by application of slurry or manure to arable or grazing land. However, concern over the risks of transmission of human viruses by the application of sewage sludge to farmland has resulted in a number of studies and useful inferences on the dangers of spreading animal wastes can be drawn from these.
- There are insufficient data available to produce an assessment in numerical terms of the risk to human, animal or plant health from the land application wastes. However by using experience, extrapolating data from other sources and analogy it is possible to construct a table of the comparative risks and this is presented at the end of Section 3 (Table 3.9) which deals with pathogens.
- The BSE saga has led to scrutiny by the public and the media of all farming practices. Increasing incidence of food poisoning, including by micro-organisms such as Cryptosporidium and E. coli 0157 which are found in organic wastes, has caused particular concerns about the risk of pathogens from the farm contaminating food for human consumption. the risks are discussed in Section 3. The management of safety depends on risk assessment but for the public at large the concepts of hazard, probability and risk are obscure jargon and the media seeks unequivocal yes/no answers which cannot be given on matters of environmental science. This is a problem which can only be dealt with by a gradual process of education.
- Part of the purpose of most treatment processes for sewage sludge is to stabilise the sludge and thereby reduce its odour potential. Often, this stabilisation is achieved by aerobic or anaerobic degradation of readily putrescible organic matter, or chemicals such as lime may be added. Whilst most (80%), and soon all, sewage sludge spread on land is treated before application, a much smaller amount of farm waste or exempt industrial waste receives treatment before landspreading. Untreated sewage sludge, septic tank sludge and blood and gut contents from abattoirs are likely to cause public nuisance from odour and gross visual pollution when spread on the land.
- The phasing out of the use on the land of septic tank sludge and blood and gut contents from abattoirs in untreated form should be considered in view of the recent decision about untreated sewage sludge which is comparable to these two exempted wastes. Landspreading of these materials is liable to cause public nuisance and there is potential for disease transmission as discussed in Section 3.
- When properly operated, advanced treatment processes for sludge such as thermal drying, lime treatment and composting, can provide sludge which is in effect free of pathogens. Most of the time, the extra cost of these processes cannot be justified on the basis of the risk from the relatively small numbers of pathogens left after conventional treatment. However, advanced treatment to produce a pathogen-free product provides extra reassurance to counter concerns about disease transmission and associated negative perceptions about the benefits of sludge recycling to land.
- For sewage sludge in particular, the problem of potentially toxic elements (PTEs) has been recognised for many years. Where sludge is used in agriculture there are statutory limits and monitoring requirements for cadmium, copper, nickel, lead, zinc, mercury and chromium; and arsenic, selenium, molybdenum and fluoride are also controlled. The problem has been reduced also by continuous efforts to restrict the entry of PTEs to the sewer, especially from industrial premises, and thus reduce levels of PTEs in sludge. The very extensive research on the environmental effects of PTEs in sewage sludge applied to land has been reviewed recently by Carrington et al. (1998) and Smith (1996). Davis and Rudd (1998) reviewed research on PTEs in relation to the landspreading of the exempted industrial wastes.
- Maximum mean annual additions of Cu and Zn of 7.5 and 15 kg ha-1 respectively over a period of 10 years are in place for sewage sludge but do not cover animal wastes. It would be prudent to set maximum metal loading rates for pig and poultry manures.
- Quantitatively, it is the detergent residues and plasticising agents which are the most significant source of organic contaminants in sludge. Linear alkylbenzene sulphonates, alkylphenols and phthalates represent respectively 84%, 10% and 3% of the total identified organic contaminant load in sludge. Whilst having a recognised toxicity hazard, these compounds degrade very quickly in the aerobic soil environment with half-lives of a few days
- Surface application of sludge to pasture land, especially permanent grassland, is a comparatively high-risk activity as regards pathogens and contaminants because of the possibility of direct ingestion of sludge residues and soil by grazing animals.
- Development of abatement technologies for ammonia emissions should be considered in view of 92 kilotonnes of ammonia nitrogen released every year in the UK from spreading of livestock manures (Philips and Pain, 1998).
- More information is needed on the quantities of 'greenhouse gases' released following the landspreading of organic wastes for comparison with emissions from other sources.
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