Domestic garden waste is collected in at Pembrokeshire County Council Civic Amenity and Recycling Centres.  In order to produce a useful compost product its important that the green waste is kept free from contaminates i.e. plastic bags and rubbish. Certain species of plants are also not permitted, such as Japanese Knot Weed and Ragwort.  As well as signs highlighting this to the users, the waste is inspected regularly and any non-conforming material is removed.  Once a sufficient quantity has been collected and all contaminates have been removed, the green waste is then loaded into shredder. This machine uses a series of hammers to pulverise the waste into fine mulch (feedstock).

Once shredded, naturally occurring micro organisms start to decompose the feedstock. Within a couple of hours it begins rise in temperature. 

The shredded feedstock is then loaded into hook-lift container trucks and transported to one of our farms.  

When the feedstock is delivered, it is inspected for a second time for any non conforming material. Providing that the feedstock is free from contaminates the load is then tipped.

The delivered feedstock is then piled up in a trapezoid shape known as a windrow. In order to maintain the composting process the micro organisms need oxygen to continue to thrive.  Laid out underneath the windrow is a perforated pipe connected to a tractor powered fan. This pipe will be used to blow air into windrow. This type of composting system is known as a static aerated pile.

Once the pile has been formed monitoring, recording and control systems are implemented. Heat is naturally generated by micro organisms with in the feedstock. Temperature is directly proportional to the biological activity within the feedstock. As the metabolic rate of the micro organisms accelerates the temperature within the system increases, conversely, as the metabolic rate of the microbes decreases, the system temperature decreases. Maintaining an average temperature of between 65c and 80c for the first 7 days (sanitation phase) favours the destruction of weed seeds, fly larvae and plant pathogens.

To ensure that these temperatures are achieved, for first week the feedstock is monitored on a daily basis.  Moisture, temperature and weather conditions are recorded; the feed stock also has to be aerated regularly through out this period.  If the temperature rises or drops out side of the control limits, the windrow is aerated using the tractor powered fan and pipe network.  Aerating the feedstock provides oxygen to the microbes which in turn increases their metabolic rate.

After the sanitation stage the compost needs to stabilise and mature. This process can take anything from 11 to 14 weeks. The compost needs to maintain an  average temperature between 45c and 80c and an average moisture content between 40% and 65%. These parameters need to be checked weekly and the results recorded. 

Once the stabilisation stage is complete the compost is screened. For application in agriculture a maximum particle size of 40mm needs to be achieved. The screener uses a rotating drum which has a mesh wrapped around to filter out over-sized particles.

Now the composted has been screened to the desired size, it is transported to the field where it will be applied. The compost will place in a pile on the edge of the field. The application of compost can commence at any time of the year as the threat of nutrient leaching is negligible. Usually the compost will be spread in the spring or autumn using a rear discharge muck spreader equipped with a feed conveyor and vertical beaters to broadcast the compost. Compost will be spread in accordance with the Environment Agency Waste Management Licensing paragraph 7, with the codes for Good Agricultural Practice, and the recommendations given in the Fertilizer Recommendations for Agricultural and Horticultural Crops (RB209) will also be adhered to.  Finished compost will be tested by a laboratory to determine nutrient content, and as a general rule, an application of 33 tonnes/hectare will supply the maximum application of 250Kg of total nitrogen