ABSTRACT
The addition of powdered activated carbon (PAC) in advanced wastewater treatment is increasingly recognized for its effectiveness in removing micropollutants from secondary effluents. This study investigates the implications of PAC dosing on treatment performance, particularly in terms of natural and effluent organic matter (OM) removal, turbidity reduction, and sludge characteristics. Results indicate that while PAC incorporation significantly enhances the adsorption of OM, it necessitates higher coagulant dosages to achieve comparable or improved turbidity levels. The study also reveals that lower pH levels facilitate the removal of OM, thereby increasing the availability of adsorption sites on PAC for micropollutants. Moreover, PAC addition results in the formation of larger, denser flocs that settle faster, leading to a reduction in sludge volume by approximately 20%. However, the increased coagulant demand and the subsequent rise in sludge volume highlight the need for the optimized process design to balance treatment efficiency with operational costs. This research provides valuable insights for the design and operation of wastewater treatment plants, emphasizing the importance of tailored coagulant dosing and process adjustments when integrating PAC into existing treatment frameworks.
HIGHLIGHTS
PAC dosing increases coagulant demand, increasing operational costs.
Coagulation enhances organic matter removal when increasing coagulant doses and lowering pH.
PAC dosing improves solid–liquid separation due to the formation of larger, denser flocs.
Sludge volumes are reduced by approximately 20% by PAC addition.
Process optimization needs: the integration of PAC dosing necessitates the optimization of process design.
INTRODUCTION
Powdered activated carbon (PAC) is used in water and wastewater treatment to remove micropollutants. Typically, PAC is continuously dosed, either before or directly into the coagulation stages, where it is incorporated into flocs and can be retained in subsequent filtration processes. The combination of adsorption and coagulation processes ensures nearly complete retention of the added PAC (Altmann et al. 2015a; Krahnstöver & Wintgens 2018), allowing treatment plants to operate efficiently and cost-effectively.
Numerous studies have confirmed that PAC retains its adsorption potential for micropollutants even when incorporated into flocs. The incorporation of PAC into flocs does not affect the adsorption process for micropollutant removal (Altmann et al. 2015b; Löwenberg et al. 2016). Both processes occur simultaneously, with dissolved organic substances being removed through adsorption on PAC and coagulation (i.e., adsorption on metal hydroxide flocs). This combination of processes is highly effective for removing these substances (Haberkamp et al. 2007; Meinel et al. 2016). However, dissolved organic matter (DOM) competes with micropollutants in the adsorption process, reducing their removal efficiency (Sontheimer et al. 1988; Zimmer et al. 1989; Müller & Uhl 2009; Worch 2021). Consequently, removing OM prior to or simultaneously with activated carbon adsorption enhances micropollutant removal by PAC. However, it must be noted that different fractions of effluent OM, such as high-molecular-weight humics, building blocks, low-molecular-weight acids, and neutrals, are removed to varying degrees by coagulation and compete differently with micropollutants (Hu et al. 2014; Altmann et al. 2015b; Wang et al. 2024).
The optimal and cost-effective removal of these contaminants requires an understanding of floc properties, how they impact removal efficiency, and how they can be influenced. Ideally, the coagulation step and the subsequent separation process (usually sedimentation and filtration) should be designed to complement each other for an effective and efficient overall treatment. Additionally, the conditions for the effective treatment of the solids (sludge) retained in the separation process should also be optimized.
The objectives of this study were to evaluate the impact of PAC dosages in the coagulation of secondary effluent on the removal of organic substances (measured as UV254 absorption), the resulting residual turbidity after sedimentation, and the sludge volume. From this, conclusions were drawn regarding necessary adjustments to coagulant doses and process stages.
MATERIALS AND METHODS
Sample collection and preparation
For the experiments, secondary effluent was collected from the municipal wastewater treatment plant (WWTP) in Hillersleben, Saxony-Anhalt, Germany. This effluent represents the inflow to an advanced wastewater treatment step aimed at further removing phosphorus and micropollutants. For comparison, water from the River Elbe (collected in Magdeburg, Saxony-Anhalt, Germany) was used to represent raw water for drinking water treatment. Samples were refrigerated at 6–8 °C until the experiments were conducted. Before each experiment, the samples were equilibrated to room temperature (22 °C).
Analytics
pH, temperature, and conductivity were measured using a multimeter (pH/Cond 3320, WTW, Germany). UV absorption at 254 nm (UV254), a proxy parameter for high-molecular-weight dissolved organic carbon (DOC; Eaton 1995; Standard Methods Committee 2023), was determined using a spectrophotometer (2100Q, Hach-Lange, Germany). DOC was measured using thermo-catalytic oxidation with a DIMATOC® 2000 analyzer (DIMATEC, Germany). Orthophosphate concentrations were determined using the standardized cuvette test (LCK 350, Hach, USA) and a spectrophotometer (CADAS 50, Dr LANGE, Germany). Turbidity was measured with a 2100Q IS Portable Turbidimeter (Hach, USA), which has a measuring range from 0 to 1,000 FNU.
Jar tests (coagulation tests)
Jar tests with and without PAC were conducted following the DVGW guideline W 218 (DVGW 1998) using a Stuart Flocculator SW6 (Bibby Scientific), which is equipped with two-blade agitators and six independently agitated jars. The energy input for mixing during coagulation was determined using a MICROSTAR 7.5 control torque stirrer (IKA-Werke, Germany). Rapid mixing of coagulants into the water was achieved using an Ultra-Turrax high dispersion instrument (IKA, Germany). The rapid stirring phase (destabilization phase) was conducted at a speed of 250 rpm (G = 375 s−1) for 1 min, followed by a 30-min slow stirring phase (flocculation phase) at 30 rpm (G < 5 s−1).
Sedimentation was carried out using Imhoff cones, with the sedimentation phase lasting 20 or 30 min. Sludge volume was not determined in all experiments due to capacity limitations.
Materials
The PAC used in the coagulation tests was supplied by Norit, with a grain size D50 of 20 μm. PAC was added as a suspension by mixing 200 mg of PAC with 10 mL of demineralized water, and it was suspended it using a magnetic stirrer. The PAC dosage per beaker was 20 mg/L, which falls within the typical range for treating secondary effluent (5–30 mg/L) (Krahnstöver & Wintgens 2018).
A 40% ferric chloride (FeCl3) solution was used as the coagulant at doses of 10, 20, 30, 40, 50, and 60 mg/L when experiments were conducted with secondary effluent and PAC. This covers a range of 0.1–0.5 mg iron per mg PAC, as reported in the review by Krahnstöver & Wintgens (2018). The turbidity of the secondary effluent was up to 15 FNU, and DOC concentrations were approximately 20 mg/L. In cases where pH adjustment was necessary, a 1.0 M caustic soda solution (Merck, Germany) was added using a burette to achieve the target pH after coagulation.
Performance evaluation
To evaluate treatment performance (i.e., the removal of particulate and dissolved compounds), samples were taken from the supernatant 3 cm below the water level using a pipette after 20 min of sedimentation. Floc growth and stability were assessed through single jar tests, where changes in apparent floc size were measured using a DEP-S probe (SEMITEC, Germany) as described by Slavik et al. (2012).
RESULTS
Particulate matter
Turbidity after coagulation with FeCl3 refers to the turbidity of the secondary effluent – mean values from five measurements with a 95% confidence interval. Blue, without PAC; red, with PAC; circle, without pH adjustment – the numbers next to data points show the pH after coagulation; triangle, with pH adjustment to 6.5–6.8.
Turbidity after coagulation with FeCl3 refers to the turbidity of the secondary effluent – mean values from five measurements with a 95% confidence interval. Blue, without PAC; red, with PAC; circle, without pH adjustment – the numbers next to data points show the pH after coagulation; triangle, with pH adjustment to 6.5–6.8.
If the pH is not adjusted, the acidity of the coagulant significantly lowers the pH compared to the secondary effluent. Higher coagulant dosages result in lower pH levels. According to Naceradska et al. (2019), the optimal pH range for particulate matter removal is between 6.5 and 6.8. Proper pH adjustment during coagulation resulted in lower turbidity compared to experiments without pH adjustment, up to a coagulant dosage of 40 mg/L. At dosages above 40 mg/L, no differences in turbidity are observed, with turbidity levels approximately half that of the secondary effluent.
Dissolved OM (as UV254)
UV254 after coagulation with FeCl3 refers to the UV254 of the secondary effluent – mean values from five measurements. Blue, without PAC; 95% confidence intervals are smaller than the sizes of the symbols. Red, with PAC; circle, without pH adjustment – the numbers next to data points show the pH after coagulation; triangle, with pH adjustment to 6.5–6.8.
UV254 after coagulation with FeCl3 refers to the UV254 of the secondary effluent – mean values from five measurements. Blue, without PAC; 95% confidence intervals are smaller than the sizes of the symbols. Red, with PAC; circle, without pH adjustment – the numbers next to data points show the pH after coagulation; triangle, with pH adjustment to 6.5–6.8.
The addition of PAC significantly enhances the removal of dissolved organic substances compared to experiments without PAC, due to the adsorption of OM on PAC in addition to adsorption on amorphous flocs. This has also been described by Huan et al. (2020). Coagulation significantly contributes to the removal of DOM, as indicated by the continued decrease in UV254 with an increasing coagulant dosage. The contribution of coagulation to overall OM removal increases with higher coagulant doses, while the contribution of PAC decreases, as evidenced by the decreasing difference in UV254 between experiments with and without PAC. At a coagulant dose of 10 mg/L, PAC contributes about 30% to DOM removal (as measured by UV254). This contribution decreases with increasing coagulant dose, remaining around 20% at dosages of 60 mg/L.
Orthophosphate
The coagulant doses used in these tests are sufficient to completely remove dissolved phosphorus present in the secondary effluent. Since the phosphorus concentration in the WWTP effluent was already below 0.3 mg P/L, theoretically only slightly more than 0.3 mg/L of iron is required to achieve a residual concentration of 0.1 mg P/L through post-precipitation or adsorption on precipitated ferric hydroxide (Siegrist & Boller 1999). After coagulation, orthophosphate concentrations were consistently below the quantification limit of 0.01 mg/L.
Floc properties
The resulting floc properties are crucial, as they influence the performance of downstream solid–liquid separation processes and are vital for selecting and designing subsequent processes. Floc properties also significantly impact the dewaterability of sludge from filter backwashing and sedimentation separation.
Course of the apparent floc size during flocculation and floc breakage caused by high turbulence in coagulation jar tests with secondary effluent; the coagulant dose was 20 mg/L; PAC dosage was 20 mg/L; values are shown as a moving average of five values.
Course of the apparent floc size during flocculation and floc breakage caused by high turbulence in coagulation jar tests with secondary effluent; the coagulant dose was 20 mg/L; PAC dosage was 20 mg/L; values are shown as a moving average of five values.
Sludge volume
Sludge volume in mL per L after a sedimentation time of 30 min from coagulation jar tests with secondary effluent; mean values from n = 3 (without PAC dosage) and n = 4 (with PAC dosage).
Sludge volume in mL per L after a sedimentation time of 30 min from coagulation jar tests with secondary effluent; mean values from n = 3 (without PAC dosage) and n = 4 (with PAC dosage).
Floating sludge formation
In secondary effluent, it was observed that very high-velocity gradients (G > 1,000 s−1) commonly used in drinking water treatment for coagulant mixing led to the formation of considerable floating sludge (scum). High-velocity gradients generate air bubbles that adhere to flocs, creating buoyant floc-microbubble aggregates, which are stabilized by extracellular polymeric substances (EPS) produced by bacteria in wastewater due to their hydrophobic sites (Müller 2006).
The DOC of the secondary effluent used in this study was approximately 18 mg/L before coagulant addition, which is significantly higher than typical DOC concentrations in raw water for drinking water treatment. As secondary effluent DOC is composed of about 50% proteins (Shon et al. 2006), mainly originating from EPS, these substances likely stabilize floating microbubble-floc compartments.
Mixing at high-velocity gradients immediately after coagulant addition is crucial for proper floc formation. If such high shear forces cause floating sludge in wastewater treatment, the mixing should be done submerged and without air. However, further investigation is needed.
CONCLUSIONS
Coagulation significantly contributes to the removal of organic substances, in addition to PAC adsorption. With similar PAC dosages, UV254, a proxy for organic substances, decreases as the coagulant dose increases. Increasing the coagulant dose reduces the PAC's load of high-molecular-weight OM, leaving more adsorption capacity for micropollutant removal.
The pH affects PAC's contribution to organic substance removal. Lower pH improves the removal of high-molecular-weight substances by coagulation, reducing PAC's contribution and increasing micropollutant adsorption capacity.
To achieve turbidity levels similar to or lower than those without PAC dosing, the coagulant dosage must be increased. This leads to larger sludge volumes, resulting in greater effort and higher costs for sludge disposal.
PAC dosing produces larger flocs that settle faster. The higher floc density due to PAC inclusion results in more effective sludge compaction, which only partly compensates for the increased amount of sludge produced due to coagulant dosage. It can be estimated that 20 mg/L of iron (III) are needed to decrease turbidity to that before addition of 20 mg/L PAC, which will produce approximately 60 mg/L of additional dry matter.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.