Sewage sludge is the thick, malodorous slurry left behind in a sewage treatment plant after its load of human and industrial chemical waste have been bio-chemically treated and the wastewater discharged. The large amount of human waste in sewage treatment plants means that the sludge contains concentration of phosphates and nitrates, desirable components of fertilizers. However, the industrial wastes that are present have highly toxic materials such as industrial solvents, heavy metals, synthetic hormones, and even radio-active waste left behind in the sludge. When sewage sludge is applied to the fields, both the nutrients and the toxic chemicals are released to the environment, and they often found at high concentrations. Sewage sludge solids comprise a mixture of organic material composed mainly of crude proteins, lipids and carbohydrates. In addition the bacteria still alive are pathogenic and contaminate soil and ground water. Sludge fertilizer is already banned in several European countries such as Switzerland, Sweden, Austria and others due to growing concern about residues in the sludge, particularly synthetic hormones and some pharmaceutical compounds.

Typical sludge comprises of about 60-80% volatile material and contains about 40% organic carbon (percent by weight of dry solids). Disposal of the sludge is expensive and normally constitutes up to 30% the total annual costs of wastewater treatment.

Numerous sludge processing options have been proposed and have the potential to convert a fraction of the organic material into usable energy. Anaerobic digestion of sewage sludge is probably the most common process employed to date, about 25% of the available organic material being converted to produce a gas rich in methane, resulting in an energy production of about 1,194 Kcal/Kg. of dry sludge solids fed to the digester.

The proposal to use the process of synthetic oil production from solid fuels for the treatment of various organic wastes, sewage sludge included is based on the similarity of the chemical composition (lipids, proteins, hydrocarbons) of the organic matter of these fuels and the waste products.

Municipal sewage sludge is gaining traction in the US and around the world as a lipid feedstock for biodiesel production. The potential energy contained in wastewater and biosolids (sewage sludge) exceeds by ten times the energy used to treat it, and can approximately meet up to 12% of the national demand. That’s enough to power New York City, Houston, Dallas and Chicago annually.

Wastewater treatments plants produce only a small quantity of the energy they need. In order to broaden new energy creation, so that every community can take advantage of the opportunity, the wastewater sector must develop and deploy new practices and technologies. Water and wastewater treatment operations have the potential to be net energy producers; they represent three percent of the total electricity consumption in the United States. The 16,583 publicly owned wastewater facilities in the United States annually produce 7.2 million metric tons of “dry solids” – biosolids with water removed.

Currently, 45 percent of the biosolids are incinerated or landfilled, and 49 percent are treated or land applied. To maximize energy from this great untapped potential would reduce sludge management costs, protect our Environment and Public Health, and improve the United States energy security. (1)

Note: (1) Energy Opportunities in Wastewater and Biosolids - Water Environment Research Foundation – 3 / 2009

Present disposal methods are inadequate

High costs and increasing regulations are the catalyst driving the wastewater sludge treatment market beyond incineration, agricultural use, landfills and ocean dumping.  Lack of available disposal sites for industrial and municipal sludge, and increasing state, national and international regulations, are driving the reuse of primary and secondary sludge as a sustainable energy source.

Pyrolysis and gasification allow recovering increased energy and products from sewage sludge compared with incineration. When sludge is burnt in a modern incineration the only practical product is heat generation, whereas the gases, oils and solid char from pyrolysis and gasification can not only be used as fuel, but also purified and used as a feedstock for petrochemicals and other applications. Obviously, there is a growing demand for sludge management technologies capable of: 1) Meeting increasingly stringent regulatory requirements, and 2) Effectively recovering material and/or energy.

A 2010 report from Lux Research projects the global sludge-to-energy market will be around $30 billion by 2020. That includes energy recovery technologies like anaerobic digestion, gasification and biofuels production. These technologies offer utilities – many of which are owned by cash-strapped municipalities – an immediate new revenue stream. Companies like BioPetrol Ltd., Enertech and Huber Technologies are all working on various ways to recover energy from sludge. (2)

Note: (2) Toxic sludge is good for you? – renewableworldenergy.com - May 2010