Biosolids as a Source of Alternate Energy

Biodrying biosolids makes it much more practical to use as an alternate fuel, by reducing the moisture content and making it much more manageable to transport and feed into a burner.

Wastewater biosolids a product that we will only see more of in our world. As we improve our wastewater processes to dump less waste into our waters, including our streams, lakes and ocean, we will generate more biosolids. Worldwide, approximately 50% is recycled for agricultural uses, and a smaller percentage is incinerated or used as an alternate fuel.

Reusing the biosolids to our soils to provide nutrients and organic matter is important for the health of our soils, however, some are concerned about some of the chemicals that we put into toilets and end up in our biosolids. Incinerating the biosolids or using them as a source of alternate fuel is also not without environmental consequence. Some are concerned about the air emissions resulting from using biosolids as a fuel.

There are a number of challenges with using biosolids as a fuel source. Firstly, its 75-80% water, which makes it expensive to transport. Secondly, biosolids are sticky and lumpy, making it difficult to feed efficiently into a burner, Thirdly, a considerable amount of the energy in the biosolids is used up in the transport and drying of the biosolids before it can be used as an alternate fuel.

We tried biodrying biosolids and made a number of observations. The ability to biodry biosolids depends on the available energy in the biosolids, which is directly correlated with the amount of processing at the waste water plant. For example, one biosolids from a small community where minimal wastewater processing occurred allowed drying of the biosolids from 77% moisture to 28% moisture in two weeks. Contrastingly, a biosolids produced from a community with more extensive processing had little available energy resulting in minimal biodrying potential!

Biodried biosolids are much easier to transport and manage, and have available energy to be used as a fuel.

The resulting dried biosolids has a calorific value similar to brown coal. It is also much more transportable and simple to feed into a burner system.

Calorific values and proximate analysis of the biosolids biodried to 28% moisture.

Additional drying of the biosolids can be achieved if we used heat recovery or heat exchange. This is done at some biodrying facilities, and is a process that can be easily added. The present drying was achieved only with the energy already present in the biosolids.