A programme of acid dosing was recently carried out at Cotton Valley WRC to help keep the significant cost of running the aeration blowers down.
Cotton Valley uses a form of high efficiency aeration with fine bubble diffuser grids, but the blowers still consume energy in excess of £700k per annum to ensure treatment of the significant flows and loadings of the site.
Any diffuser submerged in mixed liquor will eventually become covered in biological growth and organic matter.
This will gradually block the orifices and reduce the transfer of air through the diffuser and into the biomass. This then increases the back pressure, meaning the blowers have to work harder to achieve required airflow, causing overheating, premature wear and increased energy consumption. This also means poor oxygen transfer so less efficient bacterial activity, poor air pattern leading to low DO, inefficient mixing, short-circuiting, settlement etc.
Formic acid is dosed at 85% concentration to remove biological fouling. Chemical formula HCO2H, the compound also occurs naturally in many insects, and at the very low dilution ratios used in this process has no negative effect on the activated sludge biomass.
The acid is applied at a rate of roughly one litre per m3 of diffuser area via an injection nozzle inserted into the air delivery pipework. This nozzle creates an atomized spray which then comes into contact with the biological growth and breaks it down.
The use of acid dosing is a good preventative measure and is proven to reduce energy in most cases, but it is only applicable for removing biological growth – it will not remove large quantities of lime scale or certain metal deposits associated with some trade wastes. And it will unfortunately NOT dissolve grit.
“A scheme carried out very professionally, with the end results showing a huge improvement in air distribution across the aeration lanes. As-well as the energy benefits, the process treatment has been improved and the best thing about it is the whole project was carried out with-out affecting the site.” Leigh Swan, Process Optimiser
