Abstract A pilot-scale anaerobic membrane bioreactor (AnMBR) treating raw municipal wastewater was operated for approximately 600 days. This study assessed the characteristics of biosolids and membrane performance. The production of total solids (TS) and volatile solids (VS) was low, and comparable to that reported for an extended aeration system at solids residence time (SRT) longer than 40 days. The yields of TS and VS were reduced as SRT increased from 40 to 100 days. Ferric chloride (FeCl3) addition of 26 mg/L influent increased both TS and VS. The VS loading in the influent wastewater to the AnMBR was reduced by 60–82% and it was concluded the biosolids met the requirements for vector attraction reduction for land application. The nutrient content in terms of total Kjeldahl nitrogen (TKN) and total phosphorus (TP) was similar to that of anaerobically digested municipal sludges. The dewaterability of the biosolids was poorer than that reported for sludges from conventional aerobic treatment and anaerobically digested sludges. The biosolids met standards for land application with regards to heavy metals but would need further treatment to meet Class B pathogen indicator criteria.
The long-term impacts of SRT and the addition of FeCl3 on membrane performance were assessed. At lower SRT (40 days) mixed liquor TSS concentration and colloidal COD (cCOD) concentration were lowest, and capillary suction time (CST) was lowest, indicating the lowest fouling propensity. Recovery cleaning resulted in substantial reduction of resistance as indicated by both pilot plant operation and clean water flux tests. The long-term fouling rate was observed to be higher with cleaned membranes as compared to virgin membranes. The lower membrane fouling with virgin membranes suggested that accumulation of foulants resistant to cleaning caused the higher fouling rates for the cleaned membranes. The addition of 26 mg/L of FeCl3 significantly enhanced the membrane performance. For operation started with a virgin membrane the trans-membrane pressure (TMP) was maintained at values lower than 5 kPa for the first 75 days. The superior membrane performance was attributed to the reduced colloidal COD concentration and improved dewaterability.
Keywords AnMBR; Biosolids; SRT; Recovery cleaning; FeCl3 Dosing
1. Introduction Anaerobic membrane bioreactors (AnMBRs) are recognized as a sustainable technology for wastewater treatment because the anaerobic process has low sludge production, low energy requirements and can generate methane as an alternative energy source (Metcalf & Eddy Inc., 2003; Smith et al., 2012). The use of membranes for biomass separation allows long solids retention time (SRT) operation which offsets the low growth rates of anaerobic organisms while also producing a solids-free effluent (Smith et al., 2013; Gao et al., 2014). In addition, low hydraulic retention times (HRTs) can be achieved to ensure the economical viability of the AnMBRs. Therefore the AnMBR is a promising technology for municipal wastewater treatment. The application of AnMBRs to actual municipal wastewaters has been reported relatively recently (Gao et al., 2014; Huang et al., 2011; Gimenez et al., 2011; Smith et al; 2012; Lew et al., 2009; Martinez-Sosa et al., 2011). In these studies, it has been found that extending the SRT results in improved removal of COD with a concurrent increase in biogas production. Further, mixed liquor suspended solids (MLSS) concentrations increases. Long SRT operation results in a reduction in the generation of volatile suspended solids in the waste biosolids stream; however, the extent of reduction in biosolids production has not been quantified.
Other AnMBR operating strategies may also influence the properties and amount of biosolids generated in AnMBRs. For example, the addition of ferric chloride (FeCl3) to an AnMBR treating municipal wastewater (Dong et al., 2015) enhanced the removal efficiencies of COD and BOD5 because both colloidal and some soluble organic material is coagulated. The addition of FeCl3 increases the quantity of fixed suspended solids produced in the AnMBR; however, the extent of the increase has not been quantified.
Changing MLSS concentration with changing SRT, and changes in particle size distribution and the reduction of colloidal material with FeCl3 addition, likely will influence the dewaterability of the biosolids and the propensity for membrane fouling (Liao et al., 2006). MLSS concentration has been identified as a key factor in membrane fouling (Liao et al., 2006; Huang et al., 2011). It is believed that an increase in the suspended solids concentration, as measured by TSS, increases the convective flow of solids towards the membrane surface and enhances cake formation and fouling. Colloidal COD also is reported to contribute to the formation of a strongly-attached fouling layer on/into the membrane that resists physical cleaning (Choo and Lee, 1996; Fan et al., 2006; Liao et al., 2006; Dagnew et al., 2012). In the current study it was hypothesized that optimized SRT and addition of FeCl3 should improve membrane performance by reducing fouling because both of these factors change MLSS and the amount of colloidal COD in the mixed liquor.
On a related AnMBR issue, there is a lack of information in the literature on waste biosolids disposal and potential use in land application. An assessment of the feasibility for land application requires information on nutrient content (concentrations of TKN and TP), volatile solids reduction, and the concentrations of pathogens and heavy metals (McFarland, 2001; Metcalf and Eddy, 2003). Little information on these properties of the biosolids generated in AnMBRs treating municipal sewage is available.
The paper presents the impact of SRT and addition of FeCl3 on the quality and quantity of biosolids in a pilot scale AnMBR treating municipal wastewater. Membrane performance and the characteristics of mixed liquor with regard to membrane fouling also are discussed.
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