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Sludge Composting

HotRot’s fully enclosed, small footprint, composting system lends itself to composting of biosolids on-site at the wastewater or sewage treatment plant. Both primary and secondary sludge can be composted but material should be dewatered or solar-thickened as much as possible.

A HotRot sludge composting system can be integrated seamlessly into the operations of a wastewater treatment plant. HotRot also offers the ability to replace traditional organic bulking agents (shredded wood or garden waste) with a fully reusable synthetic bulker.

Sludge Composting
The Business Case

A typical wastewater treatment plant (WWTP) will produce approximately 70g of dry solids per person per day; dewatered sludge (or biosolids) normally has a moisture content of 80%, indicating that sludge production can be expected to be in the order of 350g/person/day. Even a relatively small WWTP can therefore produce significant sludge volume that requires removal and disposal.

Sludge is heavy, can cause odours and contains pathogens that can spread disease if not effectively disinfected and handled. Composting is an ideal process for turning sludge into a stable organic material that has significant benefits when applied to agricultural land.

Land disposal of sludge is becoming more restricted and costly due to environmental concerns. Alternative disposal routes are also becoming expensive and in some cases restricted because of the difficulties in processing sludge due to high moisture and potential for glass formation (during incineration), etc.

Where possible composting of biosolids should occur as close to the WWTP as possible, with the ideal being on-site. On-site composting not only reduces transportation costs but may also fit with existing permits or infrastructure for handling odorous air and other emissions. Unfortunately many WWTPs lack space or distance from neighbours to allow on-site composting using traditional composting methods. Global Composting offers a real advantage with its fully enclosed HotRot system and contractual OdourFree guarantee.

The HotRot system’s compact footprint and fully enclosed vessel ensures the system can be sited and integrated almost seamlessly with other wastewater treatment activities. Clarifier

The HotRot system also produces a highly stable product that can be stored on-site for short periods without odour risk. This means truck movements can be minimised and risks associated with trucking raw sludge or biosolids eliminated.

While sludge is an obvious candidate for composting, Global Composting has also successfully installed systems for composting sewerage grit and screenings - this process can be considered more as biological stabilisation but still offers the advantages of producing a stable odour-free product that is lighter and safer to transport.

Technical Benefits

Composting of biosolids (sludge) provides a safe useable product. Composting this material on-site at, or as near as practical to, a wastewater treatment plant (WWTP) also ensures transportation and disposal costs can be minimised.

Adding Bulkers and Amendments

Composting of sludge does require the addition of an amendment or bulker. This is typically shredded or ground wood or garden (leaf and yard) waste (sludge composting flow chart). Sourcing this material may be difficult and WWTP operators often do not consider themselves to be (or want to be) compost plant operators. Good amendment

Where this is the case Global Composting offers an integrated system for composting biosolids utilising “synthetic bulkers”. These materials are supplied with the composting plant, are automatically “dosed” with the biosolids as it enters the HotRot unit and are recovered from the compost by automatic screening. Recovered material is re-used (sludge synthetic bulker flow chart).

The integrated system is designed to fit directly in-line with a sludge dewatering plant and fits seamlessly in with other WWTP operations. Composted biosolids represent 20-30% of the weight of the input material (a 70-80% mass reduction), is biologically stable and safe for agricultural use.


Dewatering of sludge is an important consideration when looking at composting. Composting of any material generally requires a moisture content of less than 60% (wet weight basis) and a bulk density of less than 650-700kg/m3; this is generally achieved by adding amendments and bulkers but dewatering is essential to minimising the requirement for these.

Even a small difference in sludge moisture content can have a dramatic effect on the volumes of bulker required:

Take an example of sludge at 85% and 78% moisture mixed with wood chips at 25% moisture.

At 85% moisture: 1 tonne of sludge requires 0.8 tonne of bulker and the composting plant needs to process 1.8 tonne total.

At 78% moisture: 1 tonne of sludge requires 0.55 tonne of bulker.

Importantly though 1 tonne of sludge at 85% would equate to only 0.68 tonne at 78% moisture so a composting plant would need to process 1.06 tonne total, instead of 1.8 tonne. The impact of this on size and economics is obvious.

Even minimising the amount of bulker required by dewatering may still lead to a situation where obtaining bulker is difficult or expensive.

Synthetic Bulker Solution

Global Composting offers a solution using re-usable synthetic bulkers, reducing operating costs, eliminating the need for bulker handling and storage, and allowing a more automated composting system that integrates more seamlessly with existing WWTP operations.

Environmental Benefits

Composting of biosolids or sludge eliminates odours and pathogens – disease causing microorganisms – making compost from sewage sludge safe to apply agricultural land.

Composted biosolids are rich in nutrients and trace elements essential for healthy plant growth (Typical Compost Quality). Compost can also reduce farm runoff and reduces the movement of nutrients and heavy metals in the soil.

On-site composting at a waste water treatment plant (WWTP) can significantly reduce greenhouse gas emissions and the frequency of truck movements in and out of the WWTP.

Composted sludge is also dry and stable, avoiding risks associated with transport of raw material through local communities. Biosolids

A properly operated composting facility will not increase odour emissions from a WWTP and indeed the in-vessel design of a HotRot system may reduce overall odour by permitting fully enclosed materials handling.