Biological ﬁ ltration with and without prior in-line coagulation to reduce UF fouling by secondary ef ﬂ uent

The objectives of this research were to investigate bio ﬁ ltration prior to ultra ﬁ ltration (UF) for treatment of secondary ef ﬂ uent. Bio ﬁ ltration with and without prior in-line coagulation was assessed for UF membrane fouling reduction. Two parallel pilot-scale bio ﬁ lters, each with different media (sand vs. anthracite), were operated under identical conditions at a hydraulic loading rate of 0.75 m/h. A component of this investigation included the in-line application of a 1.0 mg/L dose of ferric sulfate prior to an anthracite bio ﬁ lter. All UF membrane fouling experiments were conducted at bench-scale at a constant ﬂ ux of 32 L/m 2 h (LMH). The sand (BF1) and anthracite bio ﬁ lters (BF2) removed on average 25 and 20%, respectively, of the biopolymer fraction of the ef ﬂ uent organic matter. Humic substances were less well removed at about 10%, while bio ﬁ lter in ﬂ uent turbidity was reduced by 75 and 70% through BF1 and BF2, respectively. Feeding the UF membrane with bio ﬁ lter ef ﬂ uent (no prior coagulant addition) substantially reduced both hydraulically reversible and irreversible membrane fouling by up to 60 and 80%, respectively. Hydraulically reversible and irreversible fouling were further reduced (up to 69 and 87%, respectively) by the integration of the in-line coagulation/ bio ﬁ ltration pre-treatment processes compared to bio ﬁ ltration alone. illustrates the change in the irreversible fouling rates for two different UF experiments using bio ﬁ lter ef ﬂ uent (with and without prior in-line coagulant addition). One was conducted for theusual ﬁ ltration time(24 h)fromApril21to25, 2015and the other was conducted for the extended ﬁ ltration time (48 h) from May 4 to 9, 2015. In each of the two experiments, there was a reduction in reversible fouling development when the UF was fed with the bio ﬁ lter ef ﬂ uent with prior in-line coagulant addition, however, the greater reduction was observed during the shorter (24 h) experiment (data not shown).


INTRODUCTION
While biological treatment processes are capable of substantial removal of organics found in wastewater, reusing secondary effluent requires some form of additional advanced treatment. Low pressure membrane filtration can play an important role in such additional treatment. It can produce water that may either be suitable for reuse in some non-potable applications, or be fed to downstream processes for even further treatment (e.g. nanofiltration, reverse osmosis) for other higher quality reuse applications.
Interest in the application of low pressure membranes as an advanced wastewater treatment has increased the number of research initiatives being conducted and reported upon (Haberkamp et  biofiltration as a pre-treatment to UF for secondary effluent treatment with sand as the support media. In their study, biofiltration was operated at slow sand filtration HLRs (0.25 and 0.5 m/h). Biofiltration has also been investigated as a microfiltration pre-treatment for treating secondary effluent at an EBCT of 40 min corresponding to a HLR of 0.2 m/h (Pramanik et al. ), where biological active carbon was employed to provide a higher effective surface area for biofilm development and some adsorptive capacity for EfOM removal. Investigation of the performance of biofiltration as a pre-treatment for UF for the treatment of secondary effluent with different organic composition at higher HLRs than those investigated by Zheng et al. () is necessary to demonstrate the viability of the technology.
Also, study is needed to determine if differences between non-adsorbing media (e.g. sand and anthracite) exist as it relates to the rate of removal and type of specific organic fractions that contribute most to UF fouling when treating secondary effluent.
Recently, some studies have demonstrated the effectiveness of low coagulant dosages for reducing low pressure membrane fouling (Wang & Wang ; Delgado-Diaz et al. ; Zheng et al. ; Wray et al. ). In these applications, coagulants were continuously added in-line prior to the membrane without removing particles (Wang & Wang ) or altering feed water composition (Liu & Kim ). A small footprint is required for in-line coagulation in comparison to conventional coagulation-flocculation with or without sedimentation (Zheng et al. ), and lower coagulant dosages decrease both cost and sludge production making in-line coagulation an attractive and competitive alternative. Given these observations, it is possible that a combination of in-line coagulation and biofiltration pre-treatments has the potential to better achieve two important treatment targets (fouling mitigation and high permeate quality). Azzeh et al. () reported the effectiveness of combining in-line coagulation (<0.5 mg/L alum) prior to biofiltration for reducing UF fouling treating river water (Otonabee River, Ontario, Canada). The investigation of such combinations for secondary effluent treatment by membrane filtration is needed.
The initial work in this study characterized the secondary effluent being used, including determining the concentrations of the organic fractions that have been found in previous investigations to be responsible for reversible and irreversible UF fouling. The study then investigated sand vs. anthracite biofiltration as a pre-treatment for UF as it relates to reversible and irreversible fouling reduction. The impacts of in-line coagulation prior to anthracite biofiltration on biofilter performance and the subsequent reduction of UF fouling were also examined.

Source water
The secondary effluent investigated in this study was collected from the Waterloo wastewater treatment plant (WWTP), which treats approximately 45,000 m 3 /d of predominantly domestic wastewater from about 137,000 residents in the City of Waterloo, Ontario, Canada. The raw sewage was treated by primary clarification where ferrous chloride was added for phosphorus removal. This was followed by conventional activated sludge as a biological treatment and then secondary settling tanks. The secondary effluent is UVdisinfected before being discharged into the Grand River. Secondary effluent following UV exposure was collected around 9:00 a.m. twice per week in three 200 L polyethylene drums and immediately transferred to the University of Waterloo (approximately 3.5 km from the WWTP) and stored in a holding tank. Pumping to the biofilters was immediately initiated. Water in the tank was continually mixed and allowed to increase to room temperature (23 W C).

Experimental set-up
The biofiltration set-up was constructed and operated in a wastewater pilot plant on the campus of the University of Waterloo. It consisted of two parallel glass columns 5.1 cm in internal diameter and 2.1 m high with an effective bed depth of 75 cm supported by 10 cm of (3 mm) gravel ( Figure 1). Polyethylene tanks were used for feed water storage and effluent water collection. Sand and anthracite were compared as biofilter media (BF1 and BF2, respectively). The uniformity coefficient of both media was 1.5 while the   effluent DOC was relatively hydrophilic and less aromatic (Edzwald ). There was substantial variability in TOC, turbidity, nitrate, and TKN during the investigated period.
Higher turbidities were encountered in colder water at least in the early stage of this study.   Impact of biofiltration on feed water (secondary effluent) quality A summary of organic compound and turbidity removal through the biofilters is shown in Figure 5. Reductions of 18 ± 10% (mean ± standard deviation) and 26 ± 10% in DOC and BP, respectively, were observed through BF1 while slightly lower reductions (16 ± 8 and 19 ± 12%) of

Impact of biofiltration on UF fouling
The performance of UF treating secondary effluent and bio-  Figure 6). Based on these data, it can be seen that the deposition of particles (not only BP) appear to have had an effect on UF fouling.
Hence, the TMP improvement obtained by using BF1 effluent (when compared with BF2 effluent) appears to be the result of lower particulate amounts, not differences in BP concentrations, at least for this example.

It is well known that increasing TMP during filtration
cycles is attributable to the development of hydraulically reversible and/or irreversible fouling. In the experiment illustrated in Figure 6,   reported that polysaccharides (carbon-like materials) were retained more than proteins, causing severe membrane fouling when treating biologically treated wastewater.
Since UF fouling reduction with biofiltration is attribu-   between protein content in the BP fraction and irreversible fouling of UF membranes. In these two studies, a weak correlation between BP and hydraulically reversible fouling was observed.
It is important to mention that, in the current study, when the membrane was fed with secondary effluent without pretreatment, there was no correlation between BP concentration in secondary effluent and reversible or irreversible fouling. However, irreversible fouling was correlated with the particulate matter (measured as turbidity) in secondary effluent (additional detail can be found in Aly ()).
It has been reported that the fouling mechanism of BP is primarily attributable to pore blocking ( was drawn based on the data obtained from UF experiments that were conducted to investigate the impact of biofiltration as a UF pre-treatment. It was observed that both BP and turbidity impacted membrane fouling and any increase in these compounds substantially increased reversible (data not shown) and irreversible fouling. Also, the combined impact of BP and particles on UF fouling appeared to have more impact when the membrane was fed with secondary effluent.
These observations help to explain why BP in this study were

CONCLUSIONS
This study investigated the pre-treatment of secondary effluent for reuse by biofiltration and, in-line coagulation and biofiltration in series, prior to UF. Two parallel pilot-scale  • Substantially improved UF permeate quality resulted from the integrated in-line coagulation and biofiltration processes.