Co-Composting: A method for the disposal of sludges
Co-Composting is the term used to describe the composting of a mixture of materials such as sludge cake and green waste to provide a sustainable and cost effective disposal/re-use method for the co-composted material.
Co-composting is now being adopted in the UK, where the most publicised and successful implementation has probably been by Thames Water Utilities, (Terra EcoSystems) where sewage sludge is co-composted with green waste.
With limitations now being placed on sludge disposal to land, through the implementation of EU Directives and a reduction in the amount of sewage sludge, even as a cake in solid form, which is destined to be disposed to landfill, co-composting is likely to become much more popular Europe-wide.
High quality compost products can be produced by co-composting, provided that the primary material (often known as the amendment material) is carefully selected and reasonably consistent in quality.
The term "amendment material" relates to the function of the amendment material which is to ensure that a suitably open texture is maintained in the resulting mixture of the two materials. Only by ensuring a suitable amendment material is used, and that a maximum sludge to amendment material ratio is not exceeded, can a suitable texture (providing free flow of air through the pores during composting), be guaranteed to maintain suitably controllable aerobic conditions, particularly during the initial sanitisation phase.
Even after dewatering, the sludges co-composted generally still contain in the region of a 75% moisture content, so there is a need to avoid excessive initial wetness leading to loss of structure, in sludge co-composting.
In addition to sewage sludges, co-composting is also suitable for industrial organic sludges, such as those generated by the food industry, and by brewing and fermenting activities.
Co-composting of faecal sludge (FS) has benefits for developing countries. In Ref 1 the authors state that in any co-composting plant process design it is important to establish:-
- Maximum ratio of dewatered or thickened FS in the FS/MSW (amendment material) mixture which allows for proper thermophilic composting;
- Process specifications required to ensure production of a hygienically safe compost;
- Advantages and disadvantages of static pile vs. turnable windrow composting;
- Occurrence of heavy metals in FS-derived biosolids vs. in co-compost;
- Feasible operational patterns or measures to minimise nitrogen losses during co-composting.
These are aspects which any co-composting plant designers, such as the Compost@enviros team will need to consider for all co-composting projects. For example, it will always be important to ensure that heavy metals contamination is maintained below safe thresholds, by testing in advance of process design work. Small scale laboratory trials are useful when making such assessments.
An amendment material will be selected on the basis of availability and cost. In most instances currently within the UK it should be possible to obtain suitable supplies of waste materials for use as amendment material which are substantially free of charge.
At various times the following examples of amendment materials may be suitable:
- green waste (garden waste, landscape garden waste);
- recycled wood chip;
- sawdust;
- kitchen waste - subject to ABPR compliance.
- coir (coconut husk);
- palm oil waste.
Enviros is currently working on a co-composting project in Scotland. We plan to report on this project soon. Contact us here if you would like us to consider your co-composting ideas/business plan etc.
References:
Ref 1: Co-composting of Faecal Sludge and Solid Waste, EAWAG-SANDEC/IWMI (Ghana)/French Ministry of Foreign Affairs Project Report. P.O. Box 611, CH-8600 Duebendorf, Switzerland, 2002By A. Montangero, O. Cofie S. Drescher, C. Zurbruegg and M. Strauss (2002)
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