Reverse flexing as a physical/mechanical treatment to mitigate fouling of fine bubble diffusers.

Achieving energy neutrality has shifted focus towards aeration system optimization, due to the high energy consumption of aeration processes in modern advanced wastewater treatment plants. A study on fine bubble diffuser fouling and mitigation, quantified by dynamic wet pressure (DWP), oxygen transfer efficiency and alpha was carried out in Blue Plains, Washington, DC. Four polyurethane fine bubble diffusers were installed in a pilot reactor column fed with high rate activated sludge from a full scale system. A mechanical cleaning method, reverse flexing (RF), was used to treat two diffusers (RF1, RF2), while two diffusers were kept as a control (i.e., no reverse flexing). There was a 45% increase in DWP of the control diffuser after 17 months of operation, an indication of fouling. RF treated diffusers (RF1 and RF2) did not show significant increase in DWP, and in comparison to the control diffuser prevented about 35% increase in DWP. Hence, reverse flexing potentially saves blower energy, by reducing the pressure burden on the air blower which increases blower energy requirement. However, no significant impact of the RF treatment in preventing a decrease in alpha-fouling (αF) of the fine pore diffusers, over time in operation was observed.

A survey of biofilms on wastewater aeration diffusers suggests bacterial community composition and function vary by substrate type and time.

Aeration diffusers in wastewater treatment plants generate air bubbles that promote mixing, distribution of dissolved oxygen, and microbial processing of dissolved and suspended matter in bulk solution. Biofouling of diffusers represents a significant problem to wastewater treatment plants because biofilms decrease oxygen transfer efficiency and increase backpressure on the blower. To better understand biofouling, we conducted a pilot study to survey the bacterial community composition and function of biofilms on different diffuser substrates and compare them to those in the bulk solution. DNA was extracted from the surface of ethylene-propylene-diene monomer (EPDM), polyurethane, and silicone diffusers operated for 15 months in a municipal treatment plant and sampled at 3 and 9 months. The bacterial community composition and function of the biofilms and bulk solution were determined by amplifying the 16S rRNA genes and pyrosequencing the amplicons and raw metagenomic DNA. The ordination plots and dendrograms of the 16S rRNA and functional genes showed that while the bacterial community composition and function of the bulk solution was independent of sampling time, the composition and function of the biofilms differed by diffuser type and testing time. For the EPDM and silicone diffusers, the biofilm communities were more similar in composition to the bulk solution at 3 months than 9 months. In contrast, the bacteria on the polyurethane diffusers were more dissimilar to the bulk solution at 3 months than 9 months. Taken together, the survey showed that the community composition and function of bacterial biofilms depend on the diffuser substrate and testing time, which warrants further elucidation.

Microplastics separation using stainless steel mini-hydrocyclones fabricated with additive manufacturing.

Microplastics have been widely detected in natural and engineered water systems and removing microplastics from various water matrices has become a major challenge. Mini-hydrocyclones (MHCs) have been previously applied to separate mediums of different phases. Given MHCs' capability of separating fine particles from liquid phase, three MHCs were designed and fabricated in stainless steel with 3D printing. Microplastics of densities that were both lower (<1 g·cm-3) and higher (>1 g·cm-3) than water's density were used to test the separation efficiency in ultra-purified water. The separation test was performed on single-stage MHC as well as MHCs in series in a closed hydraulic circuit. A range of important operational parameters, including split ratio, feed pressure, feed flow rate, and solid concentration, were evaluated to optimize the separation efficiency. The single-stage MHC experiment revealed that >80 % microplastics >20 µm can be effectively removed at the concentration tested, and the separation efficiency peaked at the split ratio of 35 %. MHCs in series demonstrated their ability to further enhance the separation efficiency of the ones with the same density, as well as separate microplastics of different densities. Mini-hydrocyclones' were also used to separate microplastics in synthetic stormwater, and separation efficiency reached 84 % and 98.1 % for low-density polyethylene (LDPE) and polyamide (PA). The results indicated the MHCs' potential for large-scale application in microplastic separation for industrial and municipal wastewater.

Dynamic impact of cellulose and readily biodegradable substrate on oxygen transfer efficiency in sequencing batch reactors.

Aeration is a major contributor to the high energy demand in municipal wastewater treatment plants. Thus, it is important to understand the dynamic impact of wastewater characteristics on oxygen transfer efficiency to develop suitable control strategies for minimizing energy consumption since aeration efficiency is influenced by the biodegradation of pollutants in the influent. The real-time impact of acetate as a readily biodegradable substrate and cellulose as a slowly biodegradable substrate were studied at different operational conditions. Cellulose in the influent wastewater can be removed efficiently using primary treatment technologies, such as the rotating belt filter (RBF). At an ambient DO of 2 mg l-1 and air flow of 1.02 m3h-1 (0.6 SCFM), the α-factor was more sensitive to readily biodegradable substrates than to cellulose. On average, α-factor decreased by 48% and 19% due to the addition of acetate and cellulose, respectively. At a DO of 4 mg l-1 and air flow of 1.7 m3h-1 (1 SCFM), α-factor remained constant irrespective of cellulose and acetate concentrations. Without active biomass, α-factor decreased by 47% and 43% at a DO of 2 mg l-1 (air flow of 1.02 m3h-1) and high DO of 5 mg l-1 (air flow of 1.7 m3h-1), respectively. An inverse correlation between α-factor and sCOD was defined and incorporated into a dynamic model to estimate the real-time airflow rates associated with the improvement of the oxygen transfer efficiency due to biodegradation. Finally, the RBF operated with a 158-µm mesh selectively removed cellulose, thus reducing air requirements, and energy by 25%.

Fate of cellulose in primary and secondary treatment at municipal water resource recovery facilities.

Cellulose from toilet paper is a significant fraction of particulate organics, which is recoverable. For the first time, comprehensive mapping and tracking the fate of cellulose across various unit processes at full-scale in two water resource recovery facilities located in North America and Europe was undertaken. The influent cellulose content accounted for approximately one-third of the total suspended solids (TSS). Although about 80% of the raw wastewater cellulose was removed in primary treatment, the type of primary treatment process (rotating belt filter [RBF] vs. primary clarification [PC]) had a significant impact on cellulose capture and diversion. The high cellulose content of the RBF sludge accounting for 35% of the TSS facilitates cellulose recovery. For the North American plant, with a conventional activated sludge process (SRT of 6-7 days, preceded by PC), cellulose biodegradation efficiencies of 70%-90% of the PC effluent were observed in summer and winter. For the European plant, with a modified University of Cape Town process (SRT of 14 days, without primary treatment in train 2, or preceded by RBF in train 1), comparable cellulose biodegradation efficiencies were also observed. Results from laboratory SBRs indicated that cellulose biodegradation efficiency at room temperature was 86% of the influent cellulose. PRACTITIONER POINTS: Cellulose fate was tracked across two different WWTPs in two different geographies. Cellulose in the influent wastewater accounted for 1/3 of the total suspended solids. Primary treatments were able to capture more than 80% of the influent cellulose. Cellulose was biodegraded in secondary treatment, resulting an effluent of 2-3 mg/L.

Fine-pore aeration diffusers: accelerated membrane ageing studies.

Polymeric membranes are widely used in aeration systems for biological treatment. These membranes may degrade over time and are sensitive to fouling and scaling. Membrane degradation is reflected in a decline in operating performance and higher headloss, resulting in increased energy costs. Mechanical property parameters, such as membrane hardness, Young's modulus, and orifice creep, were used to characterize the performance of membranes over time in operation and to predict their failure. Used diffusers from municipal wastewater treatment plants were collected and tested for efficiency and headloss, and then dissected to facilitate measurements of Young's modulus, hardness, and orifice creep. Higher degree of membrane fouling corresponded consistently with larger orifice creep. A lab-scale membrane ageing simulation was performed with polyurethane and four different ethylene-propylene-diene (EPDM) membrane diffusers by subjecting them to chemical ageing cycles and periodic testing. The results confirmed full-scale plant results and showed the superiority of orifice creep over Young's modulus and hardness in predicting diffuser deterioration.

Membrane properties change in fine-pore aeration diffusers: full-scale variations of transfer efficiency and headloss.

Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. Punched polymeric membranes are often used in fine-pore aeration due to their advantageous initial performance. These membranes are subject to fouling and scaling, resulting in increased headloss and reduced oxygen transfer efficiency, both contributing to increased plant energy costs. This paper describes and discusses the change in material properties for polymeric fine-pore diffusers, comparing new and used membranes. Three different diffuser technologies were tested and sample diffusers from two wastewater treatment facilities were analysed. The polymeric membranes analysed in this paper were composed of ethylene-propylene-diene monomer (EPDM), polyurethane, and silicon. Transfer efficiency is usually lower with longer times in operation, as older, dilated orifices produce larger bubbles, which are unfavourable to mass transfer. At the same time, headloss increases with time in operation, since membranes increase in rigidity and hardness, and fouling and scaling phenomena occur at the orifice opening. Change in polymer properties and laboratory test results correlate with the decrease in oxygen transfer efficiency.

Impact of fouling on the decline of aeration efficiency under different operational conditions at WRRFs.

Biofilm formation influences the most energy-demanding process in the waste water treatment cycle. Biofilm growth on the surface of wastewater aeration diffusers in water resource recovery facilities (WRRFs) can increase the energy requirements up to 50% in less than 2 years. The impact of biofilms in aeration diffusers was quantified and assessed for first time using molecular tools (i.e., Energy-dispersive X-ray, Ra and RMS and Pyrosequencing) and state-of-the-art techniques (i.e., EPS quantification, Hydrophobicity and DNA quantification). To provide a better understanding and quantitative connections between biological activity and aeration energy efficiency, two replicates of the most common diffusers were installed and tested in two different operational conditions (higher and lower organic loading rate processes) during 15 months. Different scenarios and conditions provided for first time comprehensive understanding of the major factors contributing to diffuser fouling. The array of analysis suggested that higher loading conditions can promote specialized microbial populations to halve aeration efficiency parameters (i.e., αF) in comparison to lower loading conditions. Biofilms adapted to certain operational conditions can trigger changes in diffuser membrane properties (i.e., biological enhanced roughness and hydrophobicity) and enhance EPS growth rates. Improved understanding of the effects of scaling, biofouling, aging and microbial population shifts on the decrease in aeration efficiency is provided.

Linking biofilm growth to fouling and aeration performance of fine-pore diffuser in activated sludge.

Aeration is commonly identified as the largest contributor to process energy needs in the treatment of wastewater and therefore garners significant focus in reducing energy use. Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. These diffusers are subject to fouling and scaling, resulting in loss in transfer efficiency as biofilms form and change material properties producing larger bubbles, hindering mass transfer and contributing to increased plant energy costs. This research establishes a direct correlation and apparent mechanistic link between biofilm DNA concentration and reduced aeration efficiency caused by biofilm fouling. Although the connection between biofilm growth and fouling has been implicit in discussions of diffuser fouling for many years, this research provides measured quantitative connection between the extent of biofouling and reduced diffuser efficiency. This was clearly established by studying systematically the deterioration of aeration diffusers efficiency during a 1.5 year period, concurrently with the microbiological study of the biofilm fouling in order to understand the major factors contributing to diffuser fouling. The six different diffuser technologies analyzed in this paper included four different materials which were ethylene-propylene-diene monomer (EPDM), polyurethane, silicone and ceramic. While all diffusers foul eventually, some novel materials exhibited fouling resistance. The material type played a major role in determining the biofilm characteristics (i.e., growth rate, composition, and microbial density) which directly affected the rate and intensity at what the diffusers were fouled, whereas diffuser geometry exerted little influence. Overall, a high correlation between the increase in biofilm DNA and the decrease in αF was evident (CV < 14.0 ± 2.0%). By linking bacterial growth with aeration efficiency, the research was able to show quantitatively the causal connection between bacterial fouling and energy wastage during aeration.