Hydrolysis, Sewage Sludge and Anaerobic Digestion
U K sewage sludge was once discharged to the sea from our large cities, but those days are long since gone, and ever since then, the huge volumes produced have been treated and disposed of by a variety of methods. All are expensive, and the lowest cost option of discharge to land, brings with it health and soil metals build up problems, such that its use has to be limited.
Safe and cost-effective disposal of sewage sludge is, without doubt, one of the biggest challenges now facing the wastewater industry. The high cost of energy means that some disposal routes such as incineration are less viable, and others have ceased to become uneconomic.
However, as one technology fades one other shines. That process is thermal hydrolysis, and when combined with anaerobic digestion we have an even better combination.
This has been appreciated for a long while by companies such as CAMBI, Monsal, and Veolia Water Solutions & Technologies.
Lee Mountford, (Veolia WS&T, Product Manager) has reported that almost a decade ago, Veolia Water Solutions & Technologies started work on developing a thermal hydrolysis process that could be used to pretreat thickened or dewatered sludges before biological treatment. The system is called Biothelys and it can be used in combination with aerobic processes. But put it in a process before anaerobic digestion and it has the biggest potential.
The objective of the hydrolysis process is to hydrolyse organic solids, to solubilise them, and by so doing 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 breaks up long chain cellular material into simpler chemicals, floc particles and organic macromolecules, with the added benefit that the pretreated 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 is carried out at temperatures in the range of 140-170°C, so the first step of treatment is to dewater the raw sludge to about 30%. This is done 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, 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 preheated 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 the required temperature for hydrolysis has been reached it is maintained for a preset time before the steam is released as flash steam. This is recovered for preheating another reactor. Finally, the reactor is emptied using the residual pressure inside it to aid discharge to a buffer tank. Here the hydrolysed sludge is stored and cools prior to being transferred to the anaerobic digester.
Mesophilic anaerobic digestion and final dewatering are followed by storage in covered cells for nine months. After this, the sludge is completely safe and provides an excellent compost when spread on agricultural land.
There are no mechanical rotating parts in the thermal hydrolysis reactors - mixing of raw sludge is achieved by steam injection through the treatment cycle. Hydrolysis is at a temperature of 160°C, held for at least 30 minutes.
The biogas produced in the digester is stored in a double-membrane gas holder for use as fuel in the steam generator. Excess biogas is fed to the combined heat and power (CHP) plant(s), which generate power for the 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.
The Biothelys process, together with mesophilic anaerobic digestion, at is reported to be achieving about 45-50% removal of volatile suspended solids. This is compared with 30-35% removal typically achieved by a single-stage mesophilic digester treating extended-aeration sludge.
The biogas production is reported to be increased in the same proportion. The Veolia Water Solutions & Technologies, Saumur Plant produces a biogas which is also of higher calorific value than would be expected from AD alone, at 75% methane and only about 24% carbon dioxide.
The thermal hydrolysis process can be self-sufficient in energy, using some of the biogas produced by the anaerobic digestion. The remaining biogas is used to fuel a CHP plant. In the case of larger installations, further improvements to efficiency can be made by using all the biogas in CHP plants with steam generation in a waste-heat boiler.
The AD reactor size can reportedly also be reduced when there is a good hydrolysis stage up-front. Without the thermal hydrolysis step, the digester volume would have been almost about three times that of the installed plant. This means retrofitting thermal hydrolysis as a pretreatment 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 – is reported to make them economically viable for the treatment of various sludge types, including primary, biological and blends of primary and biological sludges.
Veolia says it provides benefits in terms of volatile solids removal, better sludge dewaterability and a reduction in the quantity of sludge for disposal. The firm says it also optimises energy recovery in the form of biogas from sludge digestion, while producing a high-quality sludge that is free of pathogens, stabilised, odourless and with good fertilising characteristics - making disposal to agriculture a practicable option.
Visit Veolia Water Solutions & Technologies Biothelys web page.
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