Handling sludge: gas or slurry mixing?

Hurll Nu-Way is a supplier of continuous anaerobic digester gas mixing systems, manufactured in the UK by Utile Engineering, utilising sliding vane gas compressor technology. The company says the technology is the most efficient and cost-effective method for mixing, absorbing approximately one-third of the power required for ‘pump’ mixing.

Gas mixing requires a reliable gas compressor to work in conjunction with the most efficient and effective mixing systems. Overseas, operators have recognised that continuous unconfined gas mixing, using efficient sliding vane gas compressors, provides good results for both reliability and whole life cost.

It has meanwhile been shown that slurry (pump) mixing absorbs almost three times as much power as a sliding vane machine for the same duty point. The introduction of a small, metered amount of oil to the gas compressors may therefore be seen to be a small price to pay for the longer life, reduced maintenance, lower power consumption and noise levels, and less downtime. All these factors contribute to lower whole life cycle costs of a sliding vane mixing system.

With the increasing uptake of cogeneration, the reduced power absorbed by a sliding vane machine results in huge increases in available power that can be exported from the site to provide a significant income. The benefits to the operator are the possession of a reliable system, at low capital cost, with excellent power consumption: thus reducing carbon footprint.

It is logical to select robust and reliable equipment which requires the least amount of maintenance. The sliding vane gas compressor uses a simple design to compress the gas inside the heavy-duty ductile iron cylinder, resulting in a quiet machine that delivers pulsation-free gas. In contrast, a pump-type unit, which relies on high-velocity sludge flow through feed pipe work to mix accurately, can result in bore erosion. This, coupled with nozzle wear inside the tank, means high maintenance and additional expenses. The downtime comes with a reduction in overall mixing efficiency and, as a consequence, results in lower gas production.

Sludge pump seal failures could result in sludge on the floor, causing health and safety issues for operators as well as maintenance problems. Sludge pumps are also intolerant and susceptible to a varying sludge depth. They tend to require a stable head (fixed sludge depth) to ensure suitable mixing. Meanwhile, on-site maintenance routines for the Utile sliding vane gas compressors typically require periodical topping up of the oil tank. Checking of blades for wear is annual and can be carried out without having to dismantle the machine; a simple removal of an eyebolt is all that is needed.

In conclusion, when investing in equipment, it is important to consider not only the short-term capital costs, but both the direct and indirect whole life costs as well.