What is Biological Treatment?

Materials that were once part of a living organism will slowly rot if the conditions are right. These are called “biodegradable” wastes. Composting and anaerobic digestion are methods of controlling (and speeding up) this decomposition process.


We have a major waste challenge in Ireland. We all produce too much waste – every Irish home produces over a tonne of waste each year. Biological Treatment, which includes processes such as composting and anaerobic digestion could be the first step to meeting this waste challenge.

Separating organic waste from dry recyclables (paper, card, plastic, etc.) is an important first step in the process of reducing Ireland’s dependence on landfills.

According to a recent Environmental Protection Agency (EPA) survey, organic waste (i.e. food and garden waste) constitutes the single largest component of household waste, accounting for 36% of the total. When organic waste is mixed with other household waste, the combined waste becomes odourous, wet, and difficult to handle. Nor can it be recycled as the waste streams are mixed or contaminated. When organic waste goes into a landfill it begins to break down and creates methane (a significant greenhouse gas) and leachate (waste contaminated water), both of which require careful environmental management.


Everybody including households, food outlets, restaurants and other businesses that generate organic waste such as fruit and vegetable waste, grass clippings, hedge trimmings, etc. should biologically treat waste.

If you have a garden of any kind – you can have a compost bin. Many local authorities now provide home composting bins at subsidised rates or you can purchase one at a garden centre or hardware shop. There is no excuse – they are widely available and very easy to use. Many local authorities will also be introducing a kerbside collection of organic waste which will then be composted at centralised facilities.


If you compost your organic waste, you:

  • Reduce the amount of waste you send for final disposal. Make the rest of your waste easier to recycle.
  • Reduce the amount of organic waste going to landfills. This reduces methane gas emissions and the risk of other environmental impacts associated with landfill sites. There are also fewer attractions for pests and birds at landfills when organic waste is removed.
  • Convert your waste into valuable compost for the garden – reducing the need for artificial fertilisers

In addition,

  • Challenging targets must be achieved for the diversion of biodegradable waste from landfills.
  • Increasingly waste collection charges are based on the weight/volume of waste. Composting of your kitchen and garden waste reduces the amount of waste presented for collection, lowering charges and saving you money.


The Government is drawing up a policy setting ambitious targets to divert biodegradable waste from landfills. This will implement EU laws on landfilling (‘Landfill Directive‘) which seeks to reduce the amount of organic waste that is sent to landfills. The National Biodegradable Waste Strategy was finalised in November 2005. This policy outlines the requirement for households and businesses to employ a separate bin for food and garden waste, and this waste will have to be treated biologically (either home composting or collected for central composting/ anaerobic digestion). The policy sets a target to produce clean compost that can be used in horticulture, agriculture, forestry and other applications. Market development initiatives will be undertaken to support the development of composting. All future waste management plans will have to fully implement the Strategy.


Raw Material

The process of composting requires a continuous supply of feed in the form of organic waste. Organic material is anything that was once living. Under the correct conditions, this material will decompose into nutrient-rich compost. A list of items that can and cannot be composted can be found on the Composting page in Take Action.

Carbon to Nitrogen Ratio

although most forms of organic waste supply essential nutrients for optimum growth of microorganisms such as bacteria and fungi, they grow best with the proper level of Carbon and Nitrogen (C:N ratio). Paper, leaves and wood are high in carbon, (referred to as ‘brown’ because they are dry) while grass clippings and most fruit and vegetables are high in nitrogen (called ‘green’ because they are fresh). Decomposition speed will greatly increase with the right balance. The correct C:N ratio can be achieved by adding two parts of green material to one part of brown in the composting bin.


Turning the organic material allows more air in and serves to greatly accelerate the composting process. It ensures a constant supply of oxygen (aerobic conditions) for the bacteria to do their job. It also exposes new surfaces for bacteria to decompose and reduces the problem of odours. If you want to compost quickly, organic material should be turned once a week. In household, composting coarse materials such as cardboard and twigs will also aid aeration.


Adequate moisture levels are essential for the activity of micro-organisms. Insufficient moisture results in a reduction in decomposition by aerobic bacteria. If the composting waste is excessively wet and soggy there will be a reduction in aeration and anaerobic conditions will prevail, resulting in odour problems. The organic material should not be too dry and dusty or too wet and clumpy. A good rule of thumb is that your compost should be as moist as a wrung-out sponge.


The organic waste should not be allowed to get too cold as decomposition is greatly reduced when the temperature of the organic waste drops below approximately 13oC. Turning the organic material distributes heat evenly and speeds up the decomposition process. Composting slows down in the winter, and in the summer the material may need to be turned more regularly to keep it aerated.

Biological Organisms

Biological organisms such as bacteria, fungi, worms, and other insects, feed on and decompose the organic waste. As the temperature rises bacteria break down the organic material and their activity raises the temperature further causing the waste to decompose more quickly. Eventually, when the compost cools down, fungi, worms and insects assist the bacteria in further decomposition.


There are a number of different types of composting techniques as composting can be done in private gardens as well as in technologically advanced centralised facilities.


Individual households compost their own organic waste using homemade units or purpose-made compost bins. (See our Composting is Easy factsheet for further information).

Vermicomposting is an increasingly popular form of composting. In this method, special worms are used to break down the waste and produce a nutrient rich soil enhancer. Worms ingest their own weight in organic waste each day and excrete nutrient-rich castings.When these combine with decomposed bedding such as newspaper, food scraps or leaves, vermicompost is formed.


The majority of biowaste supplied to centralised composting facilities in Ireland is source segregated household waste. Green waste is received from council parks, landscape businesses, waste disposal contractors and the general public. Some businesses, including food manufacturers and restaurants, produce such high volumes of organic waste they create a demand for additional composting facilities. There are a number of centralised composting facilities in Ireland (see the table below). The forms of composting used at these facilities include windrow, static aerated pile and in-vessel. At present, windrow is the most preferred form of composting technology used in Ireland followed by in-vessel and aerated systems. The by-products of centralised composting are solids (compost and residues), fluids (leachate) and gas (in the form of carbon dioxide, evaporated water and ammonia). The following sections outline these processes in more detail.

Current Biological Treatment Locations in Ireland

Facility Capacity Feedstock Technology
Tralee Composting site 3,000 Household Organics Windrow
Limerick Composting Site 2,000 Household Organics In-Vessel and Windrow
Sandy Road Waste Recovery Facility 5,000 Household Organics Aerated Pile (VAR System)
Lucan Green Waste Composting 5,000 Green Waste Windrow
Aran Island Recycling Scheme 500 Household Organics In-Vessel (Biosal Unit)
Ballinasloe Composting Site 4,000 Household Organics In-Vessel and Aerated Pile (Celtic Composting)
Silliot Hill, Kildare 5,200 Commercial and Green Waste VCU In-Vessel
Kildare Sludge Plant 5,200 Municipal Sludge TEG In-Vessel
CTO Middleton 3,000 Commercial Organics Windrow
Kinsale Road Facility 2,000 Green Waste Windrow
Keady Composting Facility 65,000 Organic and Green Waste Enclosed Aerated and Windrows
McGill Facility 10,000 Commercial Sludges Enclosed Aerated
Enviro Grind Ltd. 3,000 Green Waste Windrow
Shannon Vermicomposting 1,000 Household Organics/ Municipal Sludge Windrow
Robert Delaney 10,000 Green Waste Windrow
Down District Council Composting Site 1,800 Household Organics Windrow
SimproIreland Ltd. 4,000 Green Waste Windrow
Organic Gold 3,000 Municipal Sludge, Cattle Manure Windrow
BEOFS 0 Anaerobic Digestion
Kilgreany 0 Anaerobic Digestion
Ballyshannon 0 Anaerobic Digestion


Any household can cheaply and easily begin composting in their own backyard. You can construct your own composting unit or preferably purchase a composting bin at a garden centre or hardware shop. Many Local Authorities now provide compost bins at subsidised prices.

Siting of a household composting unit

The composting unit should be situated where:

  • There is good air circulation around the bin.
  • It receives enough (but not too much) sun to ensure microbial activity will be high.
  • There is level ground and good drainage.
  • It is in a convenient location.
  • On soil or grass where worms can enter the decomposing waste, keeping it aerated.
  • Excess moisture from the organic waste can seep into the ground.

How to compost in the back garden

Layering of waste in compost bin: a diagram showing successive layers of green and brown waste on top of eachotherIt is a good idea to keep a small bin in your kitchen for convenient collection of food waste for composting. Begin by placing alternate layers, about 10 inches thick, of brown leaves, grass trimmings and suitable chopped food waste into the compost bin (see diagram). Remember to add two parts of green material to one part of brown. Some soil or old compost should also be added to ‘seed’ it with micro-organisms.

The organic waste must:

  • Be turned and mixed with a garden fork at least once every two weeks to keep the compost aerated.
  • Be kept moist, but not soggy, by watering occasionally. If the compost becomes too wet add straw, leaves or even newspaper. Ready made bins contain a lid to protect the compost against heavy rain. Make sure that the lid is not airtight preventing air circulating through the compost.

Continue to layer in an alternating pattern until the bin is three-quarters full. Under most circumstances the compost will be ready in about 1 year. Compost is ready when it becomes dark, crumbly and is uniform in texture.


Vermicomposting is a form of composting which uses worms to break down organic waste in a container called a wormery. Wormeries can be easily made at home or can be purchased purpose-made at garden centres. Vermicomposting requires more attention than ordinary composting so it should not be undertaken lightly.

How to make a basic wormery

  • Take a plastic or wooden container 30cm high x 40cm deep x 60cm wide.
  • Drill holes in the bottom and sides to allow aeration and provide for drainage.
  • Place a plastic drip tray under the worm bin.
  • Fill the container three-quarters full with suitable bedding. This can include shredded paper or other biodegradable bedding (food scraps, straw and leaves).
  • Add 1 lb of vermicomposting worms.
  • Keep a lid on the wormery.

Feed the worms about once a week. The worms eat most fruit, vegetable waste (raw or cooked), coffee, tea, egg shells, bread, etc; however citrus, dairy and meat products, fish, oil and onions should be avoided. Dog and cat litter should not be added to avoid the spread of disease. The wormery must not get too dry. The worms live beneath the surface of the organic material where it is warm, moist and dark.

The organic waste is kept aerated by the action of the worms burrowing. The worms digest their weight in organic waste each day and excrete nutrient-rich castings. The castings combined with the decomposed bedding, form vermicompost. Odour is eliminated, because the worms ingest the decomposing material.

The compost will be ready to be harvested in approximately three months. To harvest the compost, it is necessary to first remove the top 6 inches or so of fresh organic material. This layer contains most of the worms and can be stored temporarily in buckets so the finished compost underneath can be removed. When the compost is removed, transfer your fresh organic material (containing the worms) back into the wormery and start again.

The leachate produced during composting should be collected in the plastic drip tray underneath the wormery, diluted and used as a liquid fertiliser for houseplants, garden plants and vegetables.



Windrow composting is the simplest form of large-scale composting and is commonly used for green waste such as garden waste and such ‘speciality items’ as shredded Christmas trees. It is also used to compost commercial food wastes.

Using a tractor, organic waste is placed in long triangular rows, known as windrows, in the open air. Windrows are monitored to ensure optimum temperature, oxygen and moisture content are maintained. The organic waste is blended to ensure that material has the correct fabric to promote composting. The organic material is mechanically turned to distribute oxygen evenly throughout the pile and to ensure aerobic conditions. The pile is manually watered on a regular basis. This ensures that microorganisms receive an adequate supply of moisture and oxygen. As temperature increases during the first phase of composting, animal and plant pathogens which may be dangerous to humans and plants, are destroyed, ensuring the final compost is safe to use.

Windrows, especially when uncovered, are susceptible to wind and rain and need careful management to avoid problems of leachate and odours. It is recommended that the windrow be placed on a concrete pad and a leachate recycling system installed to prevent run-off of pollutants to surface or groundwater. Turning operations should also be carried out under suitable weather conditions.

In order to minimise potential problems that may arise, the following steps can be taken:

  • Food waste should be protected from easy access to avoid attracting pests.
  • Screening of the composting facilities will reduce potential negative visual impact.
  • Turn and aerate the pile regularly to avoid anaerobic decomposition which could produce gases and odours.

It takes 10 – 14 weeks to produce compost and a further 4 – 5 weeks for curing. Compost is usually screened to remove larger pieces of material, e.g. stones. The resulting compost is of very high quality and is used as potting soil for plants and as a soil improver or as a mulch for the suppression of weeds.

Static Aerated Pile

The static aerated pile was developed in an attempt to optimise the conditions that are being cultivated through the windrow method and is more technologically advanced. Green and agricultural organic waste is placed in windrow form over a series of perforated pipes. Air is forced (positive aeration) or drawn (negative aeration) through the pipes into the pile using an electric blower.

This method ensures that air is dispersed evenly throughout the pile, which maintains aerobic conditions and reduces odours. This provides optimum conditions for microbial activity and reduces excessive heat, which could reduce the effectiveness of the bacteria.

The pipes are covered with a layer of wood chips. This allows a good flow of air. The organic waste is placed over this and topped off with a 6-inch layer of peat moss. The process is referred to as ‘static’ because forced aeration eliminates the need to turn the pile. The air that is removed from the compost pile flows into a diffuser or filter pile, which contains the odours emitted from the composting material.

In-vessel System

The In-vessel system of composting is used for bio-wastes including:

  • Kitchen and garden waste
  • Animal bedding
  • Animal products
  • Canteen waste
  • Residual paper
  • Sewage sludge
  • Other organic wastes that are considered unsuitable for open air composting.

The basic principle behind in-vessel composting is the same as windrow in that it uses micro-organisms to break down organic material. This type of system, however, allows for greater control over the process as temperature, oxygen and moisture levels can be automatically adjusted. This system involves the breakdown of organic material aerobically in a closed container, rotating drum, covered or open channel, or other structures. This system can be either fully or partially enclosed.

The waste is screened for nonbiodegradable materials and then shredded into uniform particles. The uniformity of the particles will accelerate the decomposition process. The correct Carbon to Nitrogen ratio and moisture content are important. However, the composition of the organic waste is less important than for windrow or static aerated pile as the oxygen level, temperature and moisture content can be carefully controlled. This allows a greater flexibility in the mix of waste composted. The EU has recently laid down strict legislation and health rules on the management and use of animal by-products. As a country with a large dependence on agriculture it is important that we exercise due caution when dealing with activities that have a potential to impact adversely on animal health and food safety. Ireland has therefore adopted particularly stringent national legislation on the management and use of animal by-products. In pursuing the twin objectives of deveoping the necessary biological treatment capacity in Ireland and the need to maintain animal health and food safety standards, due care and consideration must be given to ensuring adherence to the appropriate national criteria.

Natural aerobic bacteria within the waste are provided with optimum conditions to break down and sanitise the waste. A computer is used to monitor and precisely control oxygen, temperature and moisture levels to accelerate decomposition.

Sufficiently high temperatures must be reached to destroy weed seeds and pathogens. This improves the quality of the compost and prevents the spread of disease. However, the temperature must not be unduly high, as this could kill the bacteria.

Pipes or blowers aerate the waste material with air being forced up through the composting waste. The in-vessel system will automatically mix and water the waste material for optimum decomposition. Odours are prevented as exhaust air is forced through a biofilter.

Any leachate produced is collected and largely reused within the system. This counters the danger of pollution of surface (rivers and lakes) and groundwaters.

Compost produced by some invessel systems must be allowed to mature in a windrow system.

As most in-vessel systems are enclosed they:

  • make little noise
  • have few pests
  • have few odour problems.

This makes them suitable for locating close to a building or in areas where space is limited.


In addition to composting, anaerobic digestion can also be used to treat some forms of biodegradable waste.

A wide range of wet and solid organic wastes such as animal slurry, livestock manures, sewage sludge, municipal organic waste, by-products from the food processing industry and milk have high polluting potential. These wastes also have the potential to produce odours. Anaerobic digestion is a highly effective and safe way of dealing with these types of wastes, producing nutrients and energy in the process.

Under controlled conditions in an airtight tank, and in the absence of oxygen (an anaerobic environment), bacteria which thrive in the absence of oxygen break down organic compounds and produce a biogas composed mainly of methane and also carbon dioxide. Methane can be used to generate electrical power, which can be fed into the national grid.

Organic waste and anaerobic bacteria are placed in the airtight tank (the digester). In order for the bacteria to survive and carry out anaerobic digestion, the following conditions must be monitored and controlled:

  • The temperature must be kept constant. although the bacteria work best at a temperature of about 35°C, generally temperatures must be adequate to ensure sanitisation of the treated materials.
  • The correct ratio of Carbon to Nitrogen must be present in the organic waste added to the digester. This ensures the proper supply of nutrient to the bacteria.
  • Neutral pH of approximately 7.0 must be provided at all times.
  • Chemicals, including those found in detergents, pesticides and treated wood must not be added to the digester.

When the above conditions are met the digestion process takes place as follows:

  1. Organic waste breaks down to sugar
  2. Decomposed material is converted to organic acids
  3. Organic acids are converted to methane gas

Biogases such as methane produced in the process are captured, stored in gas holders of various types and used to produce heat or electricity, for instance, in a Combined Heat and Power (CHP) unit. In addition, by capturing and burning the gas, the nuisance of odour is greatly reduced.

The end product of anaerobic digestion is known as the ‘digestate’ and is separated into a liquid and solid component. The solid component generally needs to be processed and then can be used as a soil enhancer as a high quality compost for golf courses, parks and market gardens. The liquid can be used as a fertiliser.

Animal slurry spread directly onto land can run-off and pollute rivers, lakes and ground water. When slurry is treated by anaerobic digestion and then spread over the land, the nutrients are retained better in the soil and are more available to plants, reducing the danger of run-off.

Anaerobic digestion also occurs naturally for example when organic waste breaks down in anaerobic zones in landfills. Methane, a greenhouse gas, is formed and emitted into the atmosphere adding to the problems of Global Warming and reduction of the Ozone layer. However, when carried out under controlled conditions anaerobic digestion is a valuable way of dealing with waste. Potentially polluting solid and liquid wastes are safely converted into ‘digestate’, while a polluting gas – methane – is safely transformed into energy.

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