Backwash efficiency evaluated based on geophysical method

In drinking water production, where sand filtration is used, the conditions of the sand filter affects the production performance and quality of the produced water. Regular maintenance includes backwash of filters by use of reversed and increased water flow and additional use of airflow to clean filter porosity for clogging materials. In Denmark, iron precipitates most frequently cause clogging of drinking water sand filters, and previous studies have shown these primarily to be situated in the upper 20cm of the sand filters.
Backwashing of sand filters are carried out by use of pure drinking water, which thereby constitutes a waste of water – today 2% of the annual pumped groundwater in Denmark is used for backwash procedures. Optimized control of backwash procedures hereby return a dual purpose: a) ensuring water quality and productivity and b) reduce water waste. However, there is a trade-off between filter growth following a too short/inefficient backwash and limiting water and energy use. Therefore, it is of outmost importance to be able to evaluate the right time and procedure for backwashing.
Today, different strategies are used to determine how often and which backwash procedure is needed. In most cases, the procedure for the backwash is adjusted as part of the start-up of a new or renovated sand filter at the waterworks. The period between each backwash is determined based on either a fixed period (days/weeks) or a specific volume of produced water (m3). At some waterworks, the pressure loss, water levels, and/or turbidity of filtered water are logged and used to determine when to backwash the filters. But how to diagnose if the filter has been backwashed sufficiently? How to determine if the start-up procedure is still the best after several years of production?
A prototype for monitoring the backwash efficiency based on geophysical methods has recently been developed. The designed equipment are looking into the upper 40cm filter bed where the main part of iron precipitates are removed and are continuously monitoring the filter condition based on conductivity of the filter material, filter porosity and water.
Full-scale equipment test were performed in an open sand filter, where data was collected continuously for six months during 2018-2019.
The prototype test demonstrates that the method are able to distinguish between well backwashed filter bed and filter bed not backwashed sufficiently for optimal production performance. The results show large potential in continuously monitoring of the filter performance, here including the specific focus on the backwash efficiency