There are approximately 20,000 private water supplies in Scotland, supplying about 3% of the population. These supplies vary in size and quality and include surface water, private impoundment reservoirs, and groundwater such as wells, boreholes and springs.
WRc was engaged by Scottish Water/Scottish Government to look into the potential of decentralised water treatment at a household level. This concept would minimise the need for building, operating and maintaining centralised water treatment plants. Instead, water with minimum treatment would be sent to the household where further treatment would be conducted at a point of entry (POE) into the property to make the water suitable for general use; additional treatment would be conducted at the point of use (under the sink) to produce a high quality water suitable for potable use. It would be essential that these systems require minimum intervention by the householders. Scheduled maintenance would be conducted by a technician.
Surface water in Scotland is often associated with high concentrations of iron, manganese, colour and total organic carbon (TOC) while containing low concentration of hardness. Treating this water is easily achievable at a water treatment plants. However, decentralised treatment of such complex water at a household level could be very challenging. This is due to the requirement for various treatment stages in order to produce water quality suitable for general use around the household and suitable for further treatment by the point of use systems.
During this study various treatment technologies were tested to assess the performance of small units commercially available for POE treatment of Scottish surface water containing an average concentration of 1000 µg/L of iron, 36 µg/L of manganese, 16 mg/L TOC and 110 mg/l Pt/Co of apparent colour. The targeted treated water quality was defined as per The Private Water Supplies (Scotland) Regulations 2006 in which the limits for iron, manganese and apparent colour are defined at 200 µg/L, 50 µg/L and 20 mg/l Pt/Co, respectively. Concentration of manganese was within the limit in the raw water while iron and apparent colour were well above the limits. There is no limit defined for TOC in the regulation.
Amongst the tested technologies, a small submerged membrane (SSM) treatment unit was tested. This unit was commercially used for storm water treatment and WRc proposed using this system for removal of Iron and Manganese at a household level. This system has a small foot print with automatic control system and minimal user intervention. While submerged membrane systems are typically used in municipal treatment plants, this application and at the household scale has not been trailed. Other adsorptive products were tested; however, they were found to be sensitive to raw water quality and flow rate, while the SSM delivered a robust product water quality independent to the variation of raw water quality.
The concept of using this system is to remove iron Fe(II) and manganese Mn(II) through increase of pH, to approximately 7.7, followed by oxidation of iron and manganese to Fe(III) and Mn(IV), via aeration, while filtering the treated water through the SSM system. Using SSM not only assisted with removal of the iron and manganese by approximately 80% and 68%, respectively, it also removed colour and TOC by approximately 35% and 22%. In this trial, Iron concentration reached below the targeted concentrations.
The performance of this system was compared against two ion exchange (IX) systems available on the market. However, these units did not prove effective in removing iron and manganese. During the trials, the presence of organics in the water, fouled the IX resin and reduce the capacity for iron and manganese absorption. Moreover, the IX systems required frequent regeneration and the discharge of the waste solution might be challenging for a household.
The SSM system not only performed in removing iron and manganese, it reduced TOC and the apparent colour which was partly due to the iron concentration and partly due to organic matter.
The SSM system has a great potential for iron and manganese removal at a household level. Particularly where the water is very soft while containing high concentrations of iron and manganese this system could be very effective. In addition this system reduces apparent colour and TOC.
The raw water used for this study was representative of some of the worst water quality experienced within Scotland. It was found that due to high concentration of TOC, additional treatment would be required after SSM. However, SSM is expected to deliver an acceptable water quality at a household level when treating water sources with lower concentrations of apparent colour and TOC.
Dr. Abraham Negaresh
Senior Water Treatment Process Engineer