Safe and cost effective disposal of sewage sludge is, without doubt, one of the biggest challenges currently facing the industry. The changing economic climate means that some disposal routes have become non-viable whilst some treatment processes, which were previously uneconomic, have now become attractive. One such process is thermal hydrolysis, and the French cities of Saumur and Chateau-Gontier chose Veolia Water Solutions & Technologies to provide this technology for their sludge treatment systems.
Making it easier to digest
Almost a decade ago, Veolia Water Solutions & Technologies started work on developing a thermal hydrolysis process which could be used to pre-treat thickened or dewatered sludges before biological treatment. Although the system, called BIOTHELYS®, can be used in combination with aerobic processes, it is before anaerobic digestion that it has the greatest economic impact. The objective of the process is to hydrolyse organic solids to solubilise them and make them more readily biodegradable. This reduces the hydraulic retention time in the downstream anaerobic digester which is, therefore, smaller and lower in capital cost. Hydrolysis disrupts cellular material, floc particles and organic macromolecules, so the pre-treated sludge is less viscous and easier to pump. In addition, the greater biodegradability of hydrolysed sludge improves removal of volatile suspended solids which increases the biogas yield and reduces the quantity of sludge for final disposal.
THERMAL HYDROLYSIS
Thermal hydrolysis is carried out at temperatures in the range 140-170°C, so the first step of treatment is to dewater the raw sludge to about 30% using a centrifuge or belt-filter in order to minimise the energy required for heating. The BIOTHELYS® system consists of two or more batch hydrolysis reactors working in parallel and out of phase with each other.
Each reactor in turn goes through a multi-step cycle taking between 120 to 165 minutes depending on the configuration. First the reactor is filled with raw sludge and this is pre-heated with recycled flash steam from the other reactor. Heating to hydrolysis temperature is completed by injecting live steam into the sludge from a steam generator fired with biogas. Once hydrolysis temperature has been reached it is maintained for a pre-set time before the steam is released as flash steam, which is recovered for pre-heating another reactor. Finally the reactor is emptied using the residual pressure in the reactor to aid discharge to the buffer tank where the hydrolysed sludge is stored prior to being cooled and transferred to the anaerobic digester.
Saumur Plant
THE SAUMUR PLANT
The Saumur wastewater treatment plant, serving a population equivalent of 60,000, is designed to treat 11,000 m3/d in two streams each consisting of a 12,400 m3 extended aeration reactor followed by a 29m diameter clarifier. The sludge is dewatered to 30%ds in a centrifuge followed by thermal hydrolysis in two vertical reactors.
Mesophilic anaerobic digestion and final dewatering are followed by storage in covered cells for nine months after which the sludge is spread on agricultural land.
There are no mechanical rotating parts in the thermal hydrolysis reactors, mixing of raw sludge being achieved by steam injection through the treatment cycle. Hydrolysis takes place at a temperature of 160°C which is held for at least 30 minutes.
Vertical reactor for thermal hydrolysis
After the BIOTHELYS® process, hydrolysed sludge is transferred to a buffer tank from which it is pumped continuously to the downstream anaerobic digester via an outlet heat exchanger which cools it to 40 to 45°C. The single 500m3 insulated anaerobic digester is a mesophilic unit operating at 35°C. It is mechanically mixed by a top-entry agitator, and there is no recirculation circuit or heating system. The digester temperature is regulated by controlling the inlet hydrolysed sludge temperature at the buffer tank outlet heat exchanger.
The biogas produced in the digester is stored in a double-membrane gasholder for use as fuel in the steam generator. Excess biogas is fed to two 20kW CHP plants which generate power for the Saumur works and also preheat the softened water feed to the steam generator. An emergency gas flare is provided for periods when the biogas consumption is less than production.
PERFORMANCE
The BIOTHELYS® process with mesophilic anaerobic digestion at Saumur is achieving about 45-50% removal of volatile suspended solids compared to 30-35% removal typically achieved by a single stage mesophilic digester treating extended aeration sludge. The biogas production is increased in the same proportion, and the Saumur biogas is 75% methane and only about 24% carbon dioxide.
At Saumur, the thermal hydrolysis process is self sufficient in energy, using some of the biogas produced by the anaerobic digestion. The remaining biogas is used to fuel a Combined Heat and Power plant. In the case of larger installations, further improvements to efficiency can be made by utilising all the biogas in CHP plants with steam generation in a waste heat boiler.
Without the thermal hydrolysis step, the digester volume would have been almost 1500m3, about three times that of the installed plant. This means that retrofitting thermal hydrolysis as a pre-treatment for existing digesters could increase their capacity by up to three times.
The reduction in digester cost resulting from systems like BIOTHELYS®, combining thermal hydrolysis with anaerobic digestion, make them economically viable for the treatment of various sludge types including primary, biological and blends of primary and biological sludges. They provide significant benefits in terms of volatile solids removal, better sludge dewaterability and a considerable reduction in the quantity of sludge for final disposal. They also optimise energy recovery in the form of biogas from biological sludge digestion whilst producing a high quality sludge free of pathogens, stabilised, odourless and with good fertilising characteristics making final disposal to agriculture a practicable option.