PROJECT DESCRIPTION
BACKGROUND
Sewage sludge generated in urban wastewater treatment plants (WWTPs) is a complex, heterogeneous mixture of microorganisms, inorganic materials and organic substances. It is classified as non-hazardous waste. Sludge disposal can represent up to 50% of the WWTP management costs. Sewage sludge must undergo some treatment in order to reduce its volume, improve its characteristics (e.g., odour elimination and organic matter content reduction) and reduce associated health problems.
The Urban Waste Water Treatment Directive (91/271/EC) stated that urban centres of more than 2 000 inhabitants should have introduced treatment of wastewater by 2005. This produced an annual increase in sewage sludge production of around 50% for the period 1992-2005 in the then EU-15. Data on sewage sludge production, however, was unavailable for several countries, while other countries had questionable figures. The highest sludge production was recorded in France, Germany, Italy, Spain and the UK, contributing to more than 60% of total sludge produced in the whole of Europe. Sewage sludge production will possibly be around 20% higher in EU-15, while a rise of 100% could be expected for new Member States. Therefore, it is important to set up solid bases for sustainable and environmentally friendly management of sewage sludge in EU in small- to medium-sized urban WWTPs, where there is a need for viable and feasible solutions for sewage sludge treatment and disposal.
OBJECTIVES
The project LIFE-ANADRY tested Dry Anaerobic Digestion (AD) technology under thermophilic (55 °C) and mesophilic (35 °C) conditions as a more effective treatment method for the sewage sludge produced in WWTPs. The implementation of dry AD of sewage sludge at semi- or pre-industrial scale had not been carried out since. The project tested this technology in a 20 m3 pilot plant to be installed in the urban WWTP of Mula (Murcia, Spain).
Tests demonstrated that the abovementioned process offers a vast improvement in terms of effectiveness, cost-effectiveness and sustainability over other methods for sludge treatment in small- to medium-sized WWTPs. The process offered:
- enhancement of biogas production with a concomitant reduction in energy use;
- reduction of the operating costs in the WWTPs;
- sludge stabilisation and hygienisation;
- reduction of carbon emissions due to the minimisation of use of inorganic fertilisers (recycling sludge as fertiliser);
- comprehensive data that supports the attractiveness of the technique for full-scale application.
RESULTS
The project was implemented in the Alguazas wastewater treatment plant (WWTP) in Murcia (Spain). The Alguazas WWTP is an urban WWTP of medium size that does not have anaerobic digestion and treats 3,500 m3/d (60,000 inhabitants) of wastewater and produces around 10 T/day of dry sludge.
The project prototype is based on a 20 m3 cylindrical reactor with a capacity of treatment of 3 T/day of dry sludge. The prototype consists of feeding system, anaerobic reactor, digestate tank and heating system. The system works as a dry anaerobic digestion, with a solids concentration about 15-20% (MS) in the influent and it has been operated with two different temperature conditions: mesophilic (35 °C) and thermophilic (55 °C). The process was also assessed under different operating conditions related to the Organic Loading Rate (OLR) and hydraulic retention time (HRT).
The project has demonstrated that dry anaerobic digestion technology working under mesophilic conditions is a feasible alternative for the management of sewage sludge in small- to medium-sized WWTPs. Moreover, the process leads to a sludge free of pathogens that can be applied in agriculture in a secure manner, reducing carbon emissions because of the minimisation of the use of inorganic fertilisers. However, the technology working under thermophilic conditions showed that the process is limited due to the partial inhibition produced by the high concentration of nitrogen species inside the digester, especially ammonia. Consequently, the process in thermophilic conditions presents a technological challenge which must be addressed in future studies.
