Summary
The EU Nitrates Directive, and the associated proposed Action Plan for Northern Ireland, has brought into sharp focus for livestock producers and the associated industries continuing up the food chain, that society requires them to act responsibly towards the environment in the way they handle manure. The Action Programme proposals have therefore put into specific regulations the requirements considered appropriate for Northern Ireland, and in this report an evaluation is made of the technologies, which could be employed to aid the industry in compliance with these requirements.
The agricultural industry in Northern Ireland has been aware for some time of the issues relating to the excess of phosphate inputs into agriculture over outputs in farm produce and stock. Reduction in the amounts of P fertiliser applied by farmers to the land could play a significant role in addressing the imbalance. The livestock sectors have already taken significant steps to reduce the phosphate content of animal feeds and the review of this topic did not reveal any significant additional steps that could be taken to improve the situation in the short term. If new sources of low phosphate feed components could be found, more use could be made of low phytate cereal varieties, or of increased phytase to allow more efficient utilisation of phytate P in rations.
While the original Action Programme included the proposal that individual farms would be required to achieve a P excess of less than 10 kg/ha by 2010 and 6 kg/ha by 2012, it was announced by Agriculture Minister Jeff Rooker on 7th July 2005, that a revised programme to be submitted to the European Commission in September 2005, would include “the need for a phosphorous balance at individual farm level will not be a requirement at this stage, but may be introduced in 2007 if a review does not show significant progress towards a reduction in the amount of phosphorous used and the introduction of commercial applications.
The overall objective is to achieve a farm P balance by 2015”. Another key element of the revised programme is “the proposed closed period for the spreading of organic manure will be from 29 October to 31 January, though this is not agreed with the Commission at this stage”.
Nevertheless, in this report, manure treatment systems are reviewed with regard to their ability to partition N and P, to reduce BOD and COD and to ease the issue of manure storage for the required over-winter periods. It is recognised that many pig farms in particular have insufficient spread-lands in relation to the 170 kg N/ha limit, let alone any P balance requirement and that the development of slurry processing facilities either on-farm or centralised that also allows nutrients to be partitioned into usable and transportable products, generate renewable energy, and results in environmentally benign outputs could be vital if livestock industries are to remain viable.
The conclusions drawn from reviewing the technologies and considering them in the light of the Nitrates Directive Action Plan are:
- A wide range of technologies are available for manure handling and processing, some of which could have significant benefits for livestock producers having to comply with the requirements of the Nitrates Directive Action Plan.
- Mechanical separation methods based on sieves, belt and screw presses generally achieve a 20% to 25% reduction of liquid volume, which may be of value if manure storage is an issue.
- These separators generally partition P or N in proportion to liquid and fibre fractions and are therefore of some value when there is a requirement to export excess nutrients from a farm.
- The decanting centrifuge, geo-textile tubes and settling basins are technologies, which, to date, have not been used in Northern Ireland for manure processing. These technologies have the potential to partition a higher proportion of P and (to a lesser extent) N in the separated solid fraction than in the liquid fraction.
- The use of chemical additives, particularly polymer flocculants, is a well established industrial technique, for precipitating solids and minerals in waste streams.
- When used with polymer flocculants and associated additives, decanting centrifuges and geo-textile tubes can achieve very high levels of partitioning of P and to a lesser extent total N.
- Decanting centrifuges can achieve high throughput of manure, but have a high capital cost.
- Static and mobile decanting centrifuge units could have potential in Northern Ireland.
- Geo-textile tubes achieve good solids and nutrient separation at low capital outlay and could have potential in Northern Ireland.
- Settling basins may be less appropriate for Northern Ireland for climatic reasons, and because there could be more odour.
- Polymers could possibly be used with other mechanical separators, but little work seems to have been conducted on this.
- In settling basins the addition of alum can significantly increase the precipitation of P.
- The addition of magnesium salts to liquid manure or separated slurry liquor will result in most P being precipitated as Struvite, which can be collected, dried, and used as a fertiliser.
- Anaerobic digestion is a mature technology which could be part of centralised or on-farm manure processing systems.
- Sustainable and economically viable establishment of Anaerobic Digestion plants is dependent on bringing together a wide range of factors into business plans.
- Anaerobic digestion plants in themselves do not deal with the issue of excess nutrients. The P and N present in the manure and other material entering the AD plant will be found in the digestate produced by the plant.
- When associated or coupled with other technologies such as centrifugal separation, AD has potential to facilitate nutrient re-distribution.
- Key issues for AD plants are - the prices obtained for electricity and heat, the gate fees obtained, the markets developed for the digestate end products, and the enlisting of public support and planning approval.
- Similar issues surround other types of ‘Turnkey’ manure processing plants.
- The specific details of any legislation will have a significant bearing on which types of systems are most likely to be economically viable.
- Water coming from processing facilities will have been derived from manure, but must be able to be used for, irrigation, washing, discharge or even as potable water if it reaches the appropriate analytical standards.
- If there is a requirement for individual farms to achieve a phosphate balance as originally envisaged in the Nitrates Directive Action Programme, then continuing efforts to reduce the P and N intake in animal diets may enable further improvements to be made, although it is recognised that the industry has already gone a long way, particularly with the reduction of P in animal diets.
