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TWIRLS: Treating Waste for Restoring Land Sustainability

Feedstock and Compost Mixes

Feedstocks and compost mixes

The ingredients of compost can be termed ‘feedstocks’. All the feedstocks that we are using are wastes created by agriculture or industry and each have unique chemical and physical properties (Table 1).

Table 1. Wastes we are using as feedstock materials for composting.

Feedstock From where? Properties
Paper pulp Paper recycling pH 8. High in carbon and calcium, low in nitrogen, phosphorus and other plant nutrients. High water holding capacity. Some copper.
Biosolids Sewage treatment works pH 8. High in nitrogen, phosphorus and calcium. Moderate levels of trace elements depending on source. High water holding capacity.
Green-waste Parks and gardens pH 7.5. High in macronutrients (nitrogen, phosphorus) and other plant nutrients. Moderate levels of copper and trace metals depending on source.
Slate waste (finely crushed) Slate quarries Neutral pH. Contains essential micronutrients (calcium, potassium, magnesium). High water holding capacity.
Sulphur Petrochemical industries Very low pH depending on application rate. No plant nutrients.
Bracken litter (Pteridium aquilinum) Bracken clearance pH 5.5. High in carbon with moderate levels of plant nutrients.
Waste soil Post-industrial sites Variable pH, trace element and contaminant levels depending on previous use. Usually low in plant nutrients but with some organic matter and carbon.
Ash Burning of wood or paper as biofuels Very high pH = 11-12. Moderate levels of trace metals depending on source of wood.

We are mixing the different feedstocks (Figure 3) to ensure that the wastes compost properly. For example, paper pulp created as a by-product during paper recycling will not compost well by itself since it contains no nitrogen and only a small amount of available carbon to provide energy for microorganisms. Adding green waste increases the growth and activity of microorganisms by providing these essential nutrients. The microorganisms are then able to decompose both the paper sludge and the green waste more efficiently resulting in a longer and hotter thermophillic phase at the outset of composting and greater loss of mass of the feedstocks overall.

Figure 3. (Left) Raw green waste collected from municipal parks and gardens in Flintshire, (Middle) de-inking paper pulp, a by-product of paper recycling, (Right) finely crushed slate waste, a by-product of quarrying slate.

We are also mixing the feedstock materials to create composts suitable for habitat restoration. To do this, it is possible to add other wastes to the finished compost to change its properties. For example, compost produced from green waste has a pH between 7 and 8 and is high in phosphorus. This is fine for using in agriculture but makes it unsuitable for growing ericaceous plant species such as heather (Calluna vulgaris). To lower the pH we will try adding acidic wastes such as sulphur, produced as a by product of the petrochemical industry, or fronds of bracken (Pteridium aquilinum) to the finished compost. To lower the availability of phosphorus, we will add finely crushed slate waste or paper pulp to the compost.