Advanced Micro-Pollutant and PFAS Removal with Micro-Sorbent and Pile Cloth Media Filtration

Introduction
For many years, researchers have sought effective methods to remove organic micro-pollutants (OMPs) from the effluents of wastewater treatment plants (WWTPs). As these methods transition from research to practical application, WWTP operators face challenges in assessing the efficiency and cost-effectiveness of various treatment processes. In Germany, regulations require an 80% reduction of OMPs in treated effluent, highlighting the urgency for effective solutions. Extensive research has focused on various OMP removal techniques, including advanced technologies such as adsorption and ozonation, both with and without filtration. This study particularly emphasizes the use of micro-sorbent (MS) technology, coagulation/flocculation (C/F), and pile cloth media filtration (PCMF). Additionally, as regulatory frameworks evolve, ongoing research is exploring the removal of PFAS compounds in conjunction with OMPs.

Background
The project involves both pilot testing and full-scale installations of micro-sorbent technology combined with C/F and PCMF. Micro-sorbents possess unique characteristics that enable a reduction in sorbent material usage by approximately 75% compared to granular activated carbon (GAC). PCMF is widely utilized globally for tertiary wastewater treatment, making the combination of micro-sorbents with specially engineered cloth media particularly effective for removing OMPs and PFAS. Following conventional biological treatments and secondary clarification, the process incorporates the addition of micro-sorbent and coagulant, a flocculation phase of 5 to 10 minutes, and subsequent PCMF filtration designed to manage increased solids loading. The removal efficiency of OMPs and PFAS is influenced by the concentrations of dissolved organic carbon (DOC) and the dosages of micro-sorbent and coagulant applied.

Method
PCMF employs an outside-in filtration technique, allowing wastewater to pass through a layer of pile cloth media (PCM), which effectively removes solids. The system cleans the pile layer without interrupting the filtration process by fluidizing it. This is achieved by drawing filtrate from the back of the filter media using a cleaning pump, thus reversing the flow direction. Cleaning is triggered when the water level in the filter tank above the PCM reaches 20 to 30 cm, or after a predetermined interval of 120 to 720 minutes between backwash cycles. Additionally, settled sludge in the tank is routinely removed every 120 to 240 minutes with a settled sludge pump. The WWTP effluent is treated by adding micro-sorbent at concentrations ranging from 5 to 15 mg/L, iron(III) chloride (FeCl3) at levels from 0 to 1900 mg/g MS, and/or anionic or cationic polymers (0 to 10 mg Polymer/g Fe3+).

To assess the adsorption of organic micro-pollutants (OMPs) and UV absorbance at 254 nm, laboratory experiments were performed using micro-sorbent. Various concentrations (5, 10, and 15 mg/L) of micro-sorbent were added to a 1-liter grab sample from the secondary clarifier effluent, with samples collected after 2 and 10 minutes of continuous mixing. These samples were analyzed for dissolved organic carbon (DOC), UV254 absorbance, and OMPs, with similar methods applied for PFAS removal.

Findings
The laboratory tests investigated how varying dosages and contact times impacted the removal of dissolved organic carbon (DOC) and organic micro-pollutants (OMPs). At a micro-sorbent dosage of 10 mg/L and a contact time of approximately 10 minutes, the removal efficiencies recorded were 40% for UV absorbance (UVA) and 25% for DOC. For a comparative analysis with PAC results, refer to Figure 2. In terms of OMP removal, the results indicated that increased dosages of micro-sorbent consistently enhanced removal efficiency, while the contact time had a negligible effect. In contrast, PAC displayed limited effectiveness for OMP removal unless the contact time exceeded 10 minutes, as detailed in Figure 3.

Pilot tests were conducted to evaluate the impact of micro-sorbent addition on the PCMF system, specifically focusing on backwash waste and recovery within the filter. The introduction of metal salts at concentrations up to 10 mg Fe3+/L (without micro-sorbent) led to an increase in backwash volume (QBW). For the PCM PES-14, the QBW rose by just 2% of the feed flow, while for UF-10, it increased to approximately 14% at a concentration of 5 mg Fe3+. This increase can be linked to the higher total suspended solids (TSS) concentration due to elevated Fe3+ levels, resulting in an increased Solid Surface Loading Rate (SLR). Conversely, dosing micro-sorbent up to 15 mg/L did not impact QBW or cleaning intervals, suggesting that the filtration process is affected by the types of solids present, not just their total mass.

Ongoing testing is examining the correlations between DOC removal, UVA removal, and PFAS removal in the WWTP. With low PFAS levels entering the facility and micro-sorbent dosages ranging from 5 to 30 mg/L, the process achieved over 70% removal efficiency, even as concentrations approached the limit of quantification (LOQ). A contact time of 5 to 10 minutes seems adequate to reach adsorption equilibrium. For additional information, refer to Figure 4.

Significance
Laboratory studies show that micro-sorbents significantly improve the removal of organic micro-pollutants and PFAS in wastewater treatment plants when combined with suitable process chemistry and PCMF technology.