Readers’ digesters

Anaerobic digesters should be familiar equipment to designers and operators alike because they have been a standard feature of WwTWs since the 1930s and the focus of yet more attention since the Kyoto agreement and carbon credits for biogas-generated electricity. However, the misconception in some design offices that a digester is just a large water tank with a lid and boiler that maintains 35°C has provided the author with several decades of commissioning work.

With AMP4 about to unleash another wave of digester building into the water industry, a short review of previous design issues is perhaps in order. More particularly, smaller single digester installations that will characterise AMP4 are less tolerant of design errors than larger installations. In the spirit of the end of term, there are some real field experiences in this review (with the sites and times suitably disguised) and without attempting to include a comprehensive pub quiz, a few test questions are provided.

A digester is a stirred, sealed tank, kept hot by means of burning biogas in a boiler. Hot water from the boiler is fed to a water-to-sludge heat exchanger. Bacteria are added to the digester contents in order to initiate a perpetual motion in which half the solids in the sludge fed to the digester are ‘eaten’ by bacteria that produce methane diluted with a little CO2 – this is biogas. Each m3 of biogas will generate 1.7kW hours of electricity in an engine plus 2kW hours of hot water, half of which is required to heat the digester. A digester typically makes one m3 of biogas per m3 of digester in the UK.

Foam issues

Anaerobic digesters foam like shaken coke bottles when shock overloading with feed sludge causes more gas to be generated than can be stripped from the sludge by the stirring system. Tolerance to foaming is one of the greatest challenges in the design of digesters. Foaming usually occurs on new plants or with new operators and can occur quickly. Its effects can be catastrophic and expensive.

Circumambulating the digester vessels were several operators dressed in yellow waders, cursing their lot. They aimed pathetically small wash-water jets at an unbelievably huge mass. This must have comprised more than 2,000m3 of foam, which changed shape in the wind but generally exhibited a spreading habit between several large digesters. The lower half of the men’s bodies was lost in the mire as they sought to locate vital equipment. Each man had a brown tide mark high on his chest indicating the foam had been significantly higher prior to my arrival – except one who cursed continuously and had no tide mark – he was the brave soul who turned off the valve to the gas bag to protect the downstream equipment, including the biogas boilers and flare. The valves had very short handles and he was not just spitting venom.

The daily logbook recorded the loading rate for the digester that was in start-up mode. Monday-Thursday 30-40m3/d, Friday 190m3 of old sour sludge. Apparently the sludge production rate from the works was higher than the start-up rate for the digester and the digester feed tank was full.

Space was required for the weekend – the best place to put the excess sludge was the digester. It was pumped across manually – all of it. The digester reacted quickly, an hour later about a third of the total contents of the digesters were deposited on the ground.

To protect the roof of a digester, gas pressure relief from the top of the digester is required. The top of digesters may look heavy but it takes only a few extra inches of water gauge to lift the mass of a large roof.

In the morning we saw the sludge coming from under the roof. The roof had been levered off by the pump pressure. There was a bit of rag in the sludge and it blocked the outlet and then the pressure relief valve (PRV).”

Flame Traps

It has been argued a source of ignition on the roof of a digester could set fire to gas escaping from a PRV and the digester headspace needs protection. Some engineers have a problem with this – the headspace of a digester has no air in it. The digester is defined as anaerobic and the fuel cannot burn or explode if it has no oxygen. The top of the digester is a controlled space with a high explosion risk gas rating, where gas may meet air. Nevertheless, increasingly, flame traps are seen under PRVs.

One neat solution to the problem of flame traps under PRVs has been adopted by at least one large water company. On a pair of PRVs the first is set to say 240mm Wg and is fitted with a flame trap under the PRV. The second is always connected in parallel to the first but has no flame trap.

This second unit is set to a higher relief pressure, say 260mm Wg, and will only operate in extremis when the first unit has failed due to blockage with foam.

There was a pause for around 2min, then the water arrived at the end of the hose – and ran back down the sleeve of the operator, no more than a dribble. Angry, frustrated curses could be heard from the top of the digester.

The acid-reducing bacteria in the digester are much more active than the methanogenic bacteria. This can lead to the production of large amounts
of CO2 during the initial
stages of an overload. The design of lines taking biogas from the top of the digester to the gas bag and service areas for boiler use, engines, gas storage and flares is a major cause of call-outs during
commissioning. These lines are often undersized and
badly engineered.

A biogas line has condensing water running on the bottom of the line, foam running on the water and gas in the space over the foam. In addition, the foam will increase the roughness of the pipe and that increases the pressure losses due to friction when the gas is moving. The implications of such a cocktail are:

Holiday cancelled. Investigation showed the main gas line was designed with an undrained ‘U’ in the line, which filled with condensing water. The flare was fed from the gas bag’s outlet. When a leak was found in the gas bag it was isolated and thus the flare was also isolated, leaving the whole system unsafe.

All the gas was now
going to the PRVs on the top of the digester. There was gas everywhere. The boiler was fed from the underside of the gas line – it was wet. The PRVs had not been unpacked properly and there was no logic to the pressure-triggering events in the control system.

It took several weeks to sort this out safely.

To some operators and designers the most beautiful thing in the built world is a well-made digester that runs for decades with little maintenance and which is robust enough to tolerate the shocks of overloading and human error with no impact on the environment except a lifetime of CO2 reduction. If you got seven out of seven questions correct, well done. May
your holidays be free of
emergency calls.