- Description of effluent
The manure produced from poultry can be very useful as an organic fertiliser. It is high in nutrients such as nitrogen, phosphorus and potassium, in fact poultry manure is higher in these nutrients than any of the other animal manures. Poultry manure will increase the amount of water the soil can hold also. Poultry manure is typically very dry and can differ depending on the nature of farming practices between systems. Poultry waste from broilers for example consists of droppings and bedding components whereas the waste from layer hens is made up of faecal matter and also grit and shells can enter the waste. Typically broiler waste will have much more dry matter and can contain spores however nutrient composition will not differ by much. Poultry manure will often be spread across agricultural fields or vegetable patches to boost soil nitrogen content increasing soil fertility and physical composition. With the increases in poultry farming and use of poultry manure as a fertiliser many problems are presented. In just one year 1000 hens can produce 65 tons of manure and poultry meat is in huge demand in Northern Ireland so it is clear that a significant treatment system is needed to process all this waste. Excess poultry manure from fields can run into nearby lakes and rivers in a process called Eutrophication. This can lead to high phosphorus levels in the water affecting aquatic life. Nutrients like nitrogen and phosphorus can cause an increase in phytoplankton which accumulate at the water surface blocking the sun light and also using up oxygen in the water. This leaves less oxygen for fish and ultimately fish can die in situations where this pollution builds up. Bugs in the water then grow anaerobically producing toxic substances like hydrogen sulphide. This process reduces the biodiversity in the water and if the water is a drinking water supply the damage is even greater. Poultry manure tends to lead to an increase in soil pH i.e. more alkaline conditions and with plants optimally growing at pH between 5 and 7 too high a pH will hinder growth so this needs to be managed too.
Disposal of animal waste including poultry waste and its use as an organic fertiliser is highly regulated by the EU (European Union) ensuring a very dilute effluent is discharged into our waterways minimising pollution. If poultry manure goes untreated it can cause serious health problems in humans as manure contains numerous pathogens. EU legislation also states that poultry manure should not be used as organic fertiliser on land which grows crops eaten raw by humans. According to the EU Urban Wastewater Treatment Directive, at least 80% of the phosphorus must be removed from the wastewater before it can enter any waterways as discharge. Regulations in England state that organic fertilisers like poultry manure cannot be spread on fields that are closer than 10 metres to any waterways like lakes or rivers. There is also distinct time periods when poultry manure can be spread, generally these time periods fall in the summer when rainfall will be low and the chances of excess manure leaching into nearby rivers and lakes is far less likely than in the winter and autumn months. Regulations such as this help to reduce levels of Eutrophication due to misuse of organic fertilisers like poultry manure.
- Potential recoverable products
With poultry manure being high in nutrients it is important to try and recover these from the waste when treating it as these nutrients can be recycled and put to use. One main nutrient present in poultry waste is phosphorus. High amounts of soil phosphorus means that when using poultry manure as an organic fertiliser more unnecessary phosphorus is being added to the soil which will inevitably enter nearby lakes and rivers. It is possible to move manure away from the farm but this can lead to large costs in transport. It has also been said that the phosphorus recovered from animal waste including poultry manure could replace a quarter of the phosphorus currently obtained from mining which is not a renewable resource and it has also been shown that phosphorus obtained from mining contains impurities (Szögi et al., 2008). With phosphorus being a life essential element it is vital to recover as much of it as possible rather than letting it go to waste and lead to pollution. The most up to date research has stated that 35.2kg of phosphorus is needed to sustain one human being for one whole year (Macintosh et al., 2018). Potassium is also present in poultry manure and if it is recovered and put through further processing potassium chloride can be created which is used in fertilizers and other chemicals.
Although the main component in poultry manure that can be recovered is nutrients like phosphorus recent developments have shown that poultry manure can be turned into electricity using Biogas plants. In 2018 a Biogas plant was built in County Antrim, Northern Ireland and is the only one of its kind in all of Europe. It is ran entirely on poultry manure using up to 40,000 tonnes a year which equivalates to only 20% of the poultry manure produced per year in Northern Ireland; the energy produced using this manure can power up to 4,000 homes (McCullough, 2018). If this Biogas plant proves sustainable, opening another two of these plants would eliminate over 50% of the poultry manure in Northern Ireland every year while powering over 10,000 homes with electricity; these are significant statistics which should not be overlooked. With global warming on the rise and drastic changes needed to preserve the planet hundreds of small steps must be taken to stop climate change and things like Biofuels and Biogases will go a long way in doing this so are becoming increasingly important.
- Description of treatment processes to be employed
There are several methods which can be employed to treat poultry manure and these include anaerobic digesters, activated sludge treatment, chemical processes, biological processes or membrane systems. In some cases a combination of these techniques may be used to treat the waste in question in order to effectively reduce nutrient levels to fit EU legislation but also to try and recover valuable products in the waste too. Anaerobic digestion involves microbes breaking down organic matter, in this case this would be poultry manure inside a sealed and heated system in the absence of oxygen and this is one option which can be utilised to process poultry manure. However, without adding another process after the digestion often the end discharge still has the same levels of phosphorus and nitrogen in it so cannot be discharged under EU legislation so the digested sludge would have to be incinerated meaning the useful nutrients i.e. phosphorus and nitrogen are completely wasted. Removing the phosphorus from the poultry manure can be done physically, chemically or biologically. It is important to note that phosphorus must be removed from wastewater as a precipitated solid unlike nitrogen which can be removed in liquid form (Heinzmann, 2004). Options to remove phosphorus physically would be centred on centrifugation or filtration techniques. Chemical treatments could be used by adding metal salts such as Iron (FeSO4, FeCl3) which will cause the phosphorus in the manure to precipitate. The precipitates can then be separated to isolate the phosphorus using methods such as filtration or sedimentation. These treatments aren’t ideal because the question of toxicity in the product means it cannot be used as fertiliser although the discharge phosphorus levels can be reduced to less than 1mg/L which would allow the discharge to enter waterways under EU legislation. However by adding another step to this anaerobic digestion process it would be possible to produce a suitable discharge and utilise the phosphorus at the same time. The production of struvite (magnesium ammonia phosphate) from poultry manure would allow the removal and subsequent utilisation of both phosphorus and nitrogen from the manure. The only down side to using struvite is the extra costs of removing excess chemicals to allow the phosphorus and nitrogen to safely be used as fertilisers under EU legislation. Even with these extra costs it seems that recovering phosphorus from digested sludge as struvite is one of the most promising ways of dealing with poultry manure. It is an experimental method of treatment and is being trialled in countries such as Italy and the Netherlands; the only country to successfully use this method is Japan and in this case the struvite has been sold to fertiliser companies (Gaterell et al., 2000, Ueno and Fujii, 2001).
Figure 1 below shows a possible method of treating poultry manure to both produce an effluent that can be discharged according to EU legislation and also recover any useful products present in the poultry manure, in this case the product in question is phosphorus. The method is a combination of anaerobic digestion and a chemical process that can use the phosphorus in the manure to produce struvite which can be used as a fertilizer in agriculture. The poultry manure enters the treatment plant firstly undergoing a preliminary treatment to remove any solids in the manure such as grit or bedding (commonly found in the waste from broiler hens) and egg shell (commonly found in the waste from laying hens). After removal of the debris from the waste it is then transferred to the next stage, the secondary clarifier where the heavy sludge settles at the bottom of the tank and the liquid component rises to the top and is removed from the tank. The phosphorus and other nutrients like nitrogen and potassium are contained within the sludge which enters the anaerobic digester. The liquid component of the waste then enters another tank where it is chlorinated; the chlorine will further disinfect the water and remove any harmful pathogens present ahead of the effluent being released into the receiving water body. The sludge then enters the anaerobic digester where it is broken down by microorganisms in the absence of oxygen eliminating any pathogens present. This process releases gases such as methane and carbon dioxide and some treatment plants will use these as biogases to generate electricity as mentioned previously. The product of this digestion then moves on to have any water present removed by filtration; the filtrate from this process then enters the struvite production plant and the sludge left over is then removed to be incinerated with the useful nutrients removed from it. The incineration process can generate energy in the form of heat. The filtrate is turned into struvite by entering a fluidised bed reactor where the liquid interacts with small granules of struvite itself allowing the formation of new struvite crystals. Any liquid left over from this process can be fed back into the system with the next batch of influent. The struvite produced can then be sold to local fertiliser companies or farms.
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By extracting at least 90% of the phosphorus in the poultry manure the mass of the dry, processed sludge going to incineration is significantly reduced (Ueno and Fujii, 2001). It is also important to note that when using the raw poultry manure as an organic fertiliser there is significant transport costs but when we take out the actual nutrients as struvite we are removing the need to transport unnecessary waste in the manure.
The proposed treatment option in Figure 1 easily meets the EU urban wastewater treatment directive exceeding the minimum percentage of phosphorus reduction by at least 10% and meeting requirement for the concentration of phosphorus in the discharge also at 1mg/L. This treatment option will significantly reduce eutrophication due to poultry manure and will also produce massive amounts of fertiliser as the demand for poultry meat continues in Northern Ireland.
- A. Szögi, M. B. Vanotti and P. G. Hunt (2008). Phosphorus Recovery from Poultry Litter. Transactions of the ASABE: 51(5), 1727-1734.
- Council Directive of 21 May 1991 concerning wastewater treatment. (1991). Official Journal of the European Communities: 135(2), 40-52.
- Gaterell, M.R., Gay, R., Wilson, R., Gochin, R.J. and Lester, J.N. (2000). An economic and environmental evaluation of the opportunities for substituting phosphorus recovered from wastewater treatment works in existing UK fertiliser markets. Environmental Technology:21, 1067-1084.
- Heinzmann, B. (2004). Phosphorus recycling in sewage treatment plants with biological phosphorus removal. Water Science and Technology: 52(10-11), 543-548.
- Macintosh, K., Doody, D., Withers, P., McDowell, R., Smith, D., Johnson, L., Bruulsema, T., O’Flaherty, V. and McGrath, J. (2018). Transforming soil phosphorus fertility management strategies to support the delivery of multiple ecosystem services from agricultural systems. Science of the Total Environment: 649, 90-98.
- McCall, W. (2018). Chicken Manure. [online] Ctahr.hawaii.edu. Available at: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/GHGS-02.pdf [Accessed 3 Oct. 2018].
- McCullough, C. (2018). Europe’s first poultry manure biogas plant in action. [online] PoultryWorld. Available at: https://www.poultryworld.net/Home/General/2018/4/Europes-first-poultry-manure-biogas-plant-in-action-272769E/ [Accessed 8 Oct. 2018].
- Stiles, W. (2018). Poultry manure management | Farming Connect. [online] Businesswales.gov.wales. Available at: https://businesswales.gov.wales/farmingconnect/posts/poultry-manure-management [Accessed 3 Oct. 2018].
- Ueno, Y. and Fujii, M. (2001). Three years experience of operating and selling recovered struvite from full-scale plant. Environmental Technology: 22, 1373-1381.
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