Reuse of activated carbon for the removal of organic micropollutants

The use of personal care products, dietary supplements and medicines is constantly increasing. This is partly due to the ageing, and growing, population. A large fraction of personal care products enter waste water directly. Dietary supplements, and to an even greater extent medicines, are broken down to only a limited extent, if at all, in the human body after ingestion. They are then excreted from the body.

Via the waste water, these substances subsequently enter sewage treatment plants (WWTPs). However, those plants are not designed to convert and/or remove these organic micropollutants (OMPs). As a result, about half of the pollutants are removed from sewage. Some of the OMPs are almost completely removed but others hardly at all, and they are then discharged into the surface water through the WWTP. This can have adverse effects on the environment and also lead to problems with drinking water supplies. Water authorities and drinking water companies, which use surface water or bank filtrate as a source, are therefore being increasingly forced to make plans for the expansion of sewage treatment or processes for the production of drinking water so that these substances will be removed effectively in the future. In addition to domestic waste water, plant protection products from horticulture are also a source of OMPs at WWTPs. In areas with extensive greenhouse horticulture, up to 25% of the influent sometimes consists of waste water from those greenhouses.

Technology

Organic micropollutants can be removed from waste water using a range of technologies. It has been shown that activated carbon administered as powdered carbon to the waste water is ideal for this purpose. However, fresh activated carbon made from coal is not a sustainable source. This project therefore looked at whether using powdered carbon that has already been used in drinking water production and powdered carbon used in lactose production in the dairy industry is suitable for the removal of OMPs from communal waste water and/or horticultural waste water.

Challenge

The available powdered carbon slurries as a residual product from drinking water production and lactose production were sampled and analysed to determine their composition, variability, and availability. Laboratory tests were conducted to study both residual adsorption capacity and the desorption of bonded compounds. That involved the use of working suspensions of plant protection products (using standard water) and organic micropollutants (based on the STOWA indicator compounds). Bioreactor tests were then performed with activated sludge to test the effect of the reused powdered carbon in treatment conditions. In addition to this practical work, an environmental impact analysis was conducted to compare the reused powdered carbon with fresh powdered carbon, the legal implications of reusing powdered carbon were considered, and a business case was prepared for the application of one of the types of carbon.

Result

This study demonstrated that the reuse of powdered carbon from drinking water production and lactose production for the removal of OMPs in a WWTP is technologically possible. The desorption of bonded OMPs to the powdered carbon from drinking water production was not seen in the conditions studied and the residual adsorption capacity is still more than adequate. The powdered carbon from lactose production is highly saturated with readily biodegradable components (lactose, vitamin B2). In the activated sludge of the WWTP, this powdered carbon is partially regenerated, restoring adsorption capacity for OMPs.

Because the reuse of powdered carbon makes it possible to avoid the production of fresh powdered carbon, the environmental benefits achieved are significant (a reduction of more than 80% compared with the use of fresh powdered carbon). Moreover, there is an interesting financial business case that provides opportunities for all stakeholders to make significant cost savings compared with the use of fresh powdered carbon. Including the reduction of carbon emissions in the price makes the business case even stronger.

In order for the powdered carbon sludge to be used as a waste product, the receiving WWTP must be licensed to collect this waste as a waste processor. For this purpose, the WWTP permit must include eural code 19.09.02 for water treatment sludge.