Under the EU Urban Wastewater Treatment Directive (UWWTD) and the Water Framework Directive (WFD), stricter requirements are soon to be imposed on the quality of effluent from wastewater treatment plants (WWTPs). In addition to better nitrogen and phosphate removal, micropollutant concentrations in the effluent must also be reduced. These micropollutants include medication residues, hormones and pesticides. Many of these substances are not removed to a sufficient extent at WWTPs. The recently completed Micropollutants Innovation Programme (Innovatieprogramma microverontreinigingen, IPMV) showed that there are several methods of effectively removing micropollutants.
One such method is the dosing of powdered activated carbon (PAC) in a water treatment plant, known as the Powdered Activated Carbon in Activated Sludge (PACAS) concept. PACAS is interesting compared to the other methods because the technique is already proven and requires little investment in conventional WWTPs. For this reason, the Foundation for Applied Water Research STOWA asked the consultancy firm TAUW to conduct a study on behalf of the water boards on experiences of, and insights into, powdered activated carbon dosing at WWTPs [1].
The water boards’ estimated PAC procurement is expected to increase sharply to around 1,200 tonnes per year by 2028. The focus of this study was on understanding everything involved in a PAC system, from the tendering process to management and maintenance. This study explored what lessons can be learned from practice based on interviews with hand-on experts.
What exactly is PAC?
Powdered activated carbon is produced in four steps: carbonisation, activation, any modification required, and post-treatment (to obtain the desired particle size distribution). The production method and raw materials used mean that the different PAC types have different properties in terms of specific gravity, adsorption capacity and safety/susceptibility to heat generation/explosion. For example, coal-based or wood-based PAC is specifically suited to the removal of micropollutants from WWTP effluent.
Use of PAC at WWTPs
A PAC system consists of a receiving silo, followed by a dosing unit from which the PAC is pumped as slurry (mixture of PAC and water) to the activated sludge tank.
In the Netherlands, PAC is already used at a number of WWTPs and also for drinking water production. Figure 1 shows the PACAS system at the Leiden North WWTP. Here, the PAC silo was installed on top of the building housing the other systems.
A number of recommendations have emerged from practice, such as that the PAC dosing system is prone to blockages. It is important to use drinking water or properly filtered process water for this purpose. Preventive maintenance on the dosing equipment to prevent blockages is also important, specifically on the vortex. In addition, the design of the PAC silo should take into account the required PAC inlet volume and the desired residual stockpile.
Explosion protection
Explosion protection measures are required when storing and dosing PAC at WWTPs, as PAC can cause a dust explosion. For this reason, PAC systems must be surrounded by zones in which only explosion-proof equipment may be used.
It is important that the electrical system of the PAC system is grounded. Introducing PAC into the system can generate static electricity. The formation of a dust layer of PAC in the working areas where the indoor components of the PAC system are located should be prevented by cleaning the relevant areas periodically.
According to the material safety data sheets (MSDS), there are no indications that the transport and/or storage of chemically activated PAC pose a higher risk than steam-activated PAC. In neighbouring countries, many WWTPs use chemically activated PAC, whereas up until now WWTPs in the Netherlands have mostly used steam-activated PAC.
PAC suppliers and use at WWTPs
PAC is mainly produced in Asia and the United States, but both steam-activated and chemically activated PAC are also produced in Europe. Since PAC is traded on a global level, there is enough non-fossil PAC to meet demand if all Dutch WWTPs are equipped with PACAS systems. In other European countries, especially Germany and Switzerland, PAC is already used at dozens of WWTPs for the removal of micropollutants. Both steam-activated and chemically activated PAC are used.
Table 1 contains information from interviews with five PAC suppliers. The prices quoted by PAC suppliers for fossil and non-fossil PAC at the time of writing were 2 to 2.5 euros/kg for fossil PAC and 3 to 3.5 euros/kg for non-fossil PAC. The PAC price can fluctuate depending on the energy price. For example, the high gas price in 2022–2023 caused the price for fossil PAC to rise to around 4 euros/kg.
Leaching of salts and other substances from PAC
Previous STOWA research suggests that using PAC from coal leads to an increase in sulphur, potassium and sodium in the sludge [2]. Potassium and sulphur concentrations were 22% and 19% higher, respectively, than at the reference WWTP without PAC dosing. A key question was whether the elevated potassium and sodium concentrations were only observed in the sludge or whether leaching also occurred to the water. In that case, the WWTP effluent would contain elevated concentrations of potassium and sodium, which would be highly undesirable.
Monitoring was therefore carried out in STOWA 2023-02 to identify any leaching of metals from the sludge to the aqueous phase. No elevated concentrations of metals were found in the aqueous phase, so it can be assumed that the metals were captured in the sewage sludge. This sludge was disposed of and processed separately. However, the potential presence of metals in PAC is a point for attention, as increased concentrations of metals in the sludge can affect the final treatment process and the reusability of the ash remaining after incineration.
The leaching of small particles of powdered activated carbon through the effluent to surface water has been investigated at several WWTPs but cannot be easily quantified. Research at the Simpelveld WWTP suggests that the PAC concentration in the effluent is <1 mg/l (detection limit) [1].
Management and maintenance
Managing and maintaining a PACAS system takes an average of one hour a day. The scale of the system makes little difference in this respect, as scale has a very limited impact on the number of components in the system. To prevent blockage of the sprinkler system, it is important that the water used is free of particles. It is also important to maintain and manage the system properly.
Sustainability
The sustainability of PAC can be assessed based on its raw materials. We make a distinction between fossil PAC (coal-based) and non-fossil PAC. Examples include PAC produced from wood, PAC released from reactivating granular coal, mixtures containing PAC, and recycled PAC from drinking water production and industrial processes. PAC is also produced from a combination of raw materials, for example a mix of raw materials from fossil and non-fossil sources. Sustainability can be assessed based on the origin of the raw materials, their carbon footprint and the production of the PAC.
Table 2 provides information on the carbon emissions of different types of PAC. This shows that the non-renewable carbon footprint of fossil PAC is significantly higher than that of non-fossil PAC. This can be explained by the fact that coal as a raw material itself does not represent carbon emissions.
Tendering process for PAC
A tendering process allows PAC to be assessed based on its micropollutant removal efficiency. PAC can also be assessed on the basis of the origin of its raw materials (inside or outside the EU, for example) and a variety of physical and chemical characteristics, such as particle size, moisture content, explosion-related parameters, security of supply and references. All these PAC properties can be considered in the tendering process. This can be done through various forms of invitations to tender, such as ‘most economically advantageous tender’ (MEAT) or ‘best price-quality ratio’ (BPQR).
Product developments
An interesting development is the reuse of PAC from drinking water production. AquaMinerals is currently working with the Rijnland water board, Aa en Maas water board and Dunea to explore the possibility of using saturated PAC from drinking water production at the Gouda WWTP [4]. A potential advantage of this is that the product is already supplied as a slurry, eliminating the need for a silo and complex dosing equipment. A simple container with mixer and dosing pump is sufficient. There are also no explosion risks and costs may be lower.
Recommendations
It is recommended that chemically activated PAC is not excluded in advance in a tendering process, but that a target is incorporated for removal efficiency as well as product requirements for aspects such as explosion protection.
There is limited information on the possible leaching of substances from PAC to the aqueous phase. It is recommended that leaching tests are performed on different types of PAC. It is also recommended that a follow-up study is conducted to determine the composition of different types of PAC.
There is currently no useful method for quantifying PAC leaching, but developments such as thermogravimetric analysis may offer a solution [2]. Further research into this is recommended.
Stricter requirements are soon to be imposed on the effluent quality of WWTPs. For example, many WWTPs will be required to remove 80% of micropollutants, including medication residues. One method of removing micropollutants involves powder activated carbon (PAC) dosing. Based on interviews with hands-on experts, this study explored what lessons can be learned from practice in terms of operation, sustainability, management, maintenance and the tendering process for PAC systems.