NB – this is the text of a case study produced by Anglian Water for internal distribution under their Drop CO2 Hawk programme. Reproduced by kind permission.
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 landings of the site.
Project stats: costing and savings figures appear here. Please contact us for the detail; savings were 92MWh, 50 tonnes CO2.
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 the 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 dilutions 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 cubic metre of diffuser area via an injection nozzle inserted into the air delivery pipework. This nozzle creates an atomised 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 is only applicable for removing biological growth – it will not remove large quantities of limescale 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 without affecting the site” – Leigh Swan, Process Optimiser