Dry anaerobic digestion is an alternative management and treatment solution for sewage sludge
Good sludge management is a relevant part of the wastewater treatment process. Huge amounts of sludge are produced and it´s necessary to find a good solution to deal with it. Usual systems are compostage, thermal dry, incineration, landfill or soil application, and the main drivers to choose the technology are the destiny requirements, viability and the safety, moreover environmental issues.
Dry anaerobic management is a promising system when we want improve the quality of the product, to use as amendment, reducing the pathogenic load, stabilizing the sludge and, at the same time, to produce biogas. It could be used for small and medium plants, with non-digested sludge.
The process consists of a digestion treatment in a tank, where we introduce dehydrated sludge (12- 14 % dryness). We have the sludge in mesophilic conditions for 40 days and the volatile organic matter of the sludge is transformed in biogas and the sludge is disinfected. Using the biogas in an engine we can produce electricity and heat to maintain the mesophilic conditions (sometimes is necessary to apply more energy, depending of the outdoor temperature).The energy to mix the sludge is very low.
At the end of the process we get a stabilized sludge, without pathogens and with a high capacity to use as amendment and fertilizer in agriculture.
The main interest groups are farmers, for getting an appreciated product, plant operators, for finding a viable system to manage properly a waste.
This action is within a Life Project (LIFE-ANADRY), supported by several organizations and with European funds.
If the feasibility study is led by a Public Adm, can serve as a general conclusion document for implementing the technology assessing some possible scenarios.
Evidence of success
The main advantage consists of treating this sludge with a total solids 20-40% compared to 10% for wet digestion, reducing the transport (reducing 192 tons CO2/year of GE) and the digester size.
Production of biogas (reducing in the WWTP energy consumption, F≈ 25 m3/d; CH4 >60%).
Possibility of operating at low retention times (10-30 d) treating a greater quantity of sludge.
Sludge stabilization and disinfection, reduction of Volatile Solid >35% and pathogens>90% E. coli and salmonella.
Potential for learning or transfer
Organic matter and nutrients are much appreciated products in many regions where agriculture has a strong implementation. Sludge of the WWTP has some of them and to be able to offer a quality and safe product with these characteristics is relevant. If we consider that this product is rich in phosphorus, and the worldwide reserves of this mineral are lowering very fast, is crucial to recover this product as much as possible.
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