Elsevier

Bioresource Technology

Volume 175, January 2015, Pages 436-444
Bioresource Technology

Recovery of volatile fatty acids from fermentation of sewage sludge in municipal wastewater treatment plants

https://doi.org/10.1016/j.biortech.2014.09.107Get rights and content

Highlights

Abstract

This work investigated the pilot scale production of short chain fatty acids (SCFAs) from sewage sludge through alkaline fermentation and the subsequent membrane filtration. Furthermore, the impact of the fermentation liquid on nutrient bioremoval was examined. The addition of wollastonite in the fermenter to buffer the pH affected the composition of the carbon source produced during fermentation, resulting in higher COD/NH4-N and COD/PO4-P ratios in the liquid phase and higher content of propionic acid. The addition of wollastonite decreased the capillary suction time (CST) and the time to filter (TTF), resulting in favorable dewatering characteristics. The sludge dewatering characteristics and the separation process were adversely affected from the use of caustic soda. When wollastonite was added, the permeate flux increased by 32%, compared to the use of caustic soda. When fermentation liquid was added as carbon source for nutrient removal, higher removal rates were obtained compared to the use of acetic acid.

Introduction

Wastewater facilities are increasingly required to implement treatment process improvements to meet stricter discharge limits with respect to nutrients and at the same time want to maintain low operating expenses. Often commercially available organic carbon sources are used in wastewater treatment plants (WWTPs) for biological nutrient removal (BNR) from wastewater. However, the price of commercial synthetic carbon sources has significantly increased over the last years. Therefore, the use of non-conventional carbon sources seems to be an appealing alternative, considering their lower greenhouse gas (GHG) footprint compared to conventional ones. Fermenting sludge to produce short chain fatty acids (SCFAs) that can then be used in the subsequent BNR process seems to be a sustainable process, having the additional advantage of decreasing the amount of sludge to be disposed (Li et al., 2011, Tong and Chen, 2007, Ucisik and Henze, 2008, Zheng et al., 2010). Recovering SCFAs from sewage sludge is currently a challenge in WWTPs. Depending on process conditions of the sewage sludge fermentation, the concentration of SCFAs can be satisfactory (i.e. >2 g/L), with favorable proportions of acids for the subsequent BNR process (i.e. ∼40% acetic acid (HAc), ∼15% propionic acid (HPr), ∼25% butyric acid (HBut)) (Jiang et al., 2009, Lee et al., 2014, Li et al., 2011). The results of recent studies have shown that the use of sludge fermentation liquid (SFL) decreases the production of nitrous oxide (N2O) and nitric oxide (NO) during the BNR processes accomplished via the nitrite pathway (Zhu and Chen, 2011). Despite the extensive work on sludge fermentation, there is still a gap of knowledge about how process parameters (i.e. pH, temperature, solids retention time (SRT), hydraulic retention time (HRT)) can affect the efficiency of sludge fermentation in terms of the composition and the quantity of SCFAs. Most applications of waste-derived SCFAs still remain at laboratory scale (Lee et al., 2014). To ascertain the transferability of the process from the laboratory to the market, pilot scale studies are also required.

The pH is an important factor that controls the hydrolysis and acidification during sewage sludge fermentation. Alkaline conditions are expected to lead to more soluble protein and carbohydrate generation. At the same time, the methanogenic activity is inhibited resulting in enhanced SCFAs production (Wu et al., 2009). The optimum pH has been found to be in the range of 9–11 and can be maintained with the use NaOH and Ca(OH)2 (Su et al., 2013, Wu et al., 2009). However, this practice is not desirable in full-scale plants, since it increases the cost and the environmental impact of the process. The addition of alkaline silicate minerals to control the pH can increase the sustainability of the fermentation process. The economic evaluation concerning the use of biowaste derived external carbon sources revealed that the organic fraction of municipal solid waste (OFMSW) fermentation liquid (FL) is a less expensive option compared to the use of synthetic HAc for the treatment of high strength nitrogenous effluents (Katsou et al., 2014). Considering the complex physicochemical nature of fermented effluents, the separation of SCFAs from fermented sludge is not a straightforward process. There are very few studies examining the solid/liquid (S/L) separation of fermented effluents. The effective and straightforward separation of FL from the fermented sludge can enhance the sustainability of the process and can lead to reduced production of wet sludge. In previous studies, the sludge dewatering ability was negatively affected by the use of caustic soda due to the release of Na+, which impaired the separation of the FL from the fermented sludge (Su et al., 2013). One commonly applied technique for S/L separation is filtration. Nevertheless, membrane filtration is energy demanding, which limits its applications (Zhang et al., 2009). S/L separation is not always effective and the deteriorated filtration capability of the sludge makes the conventional sludge dewatering methods not practical (Tong and Chen, 2007, Yuan et al., 2006).

Within this context, the application of an efficient separation method based on membrane filtration is examined, aiming at the optimization of the whole treatment scheme. Membrane technology has been studied for the separation, isolation, recovery and utilization of volatile fatty acids (VFAs) (Zacharof and Lovitt, 2012). Membrane filtration is expected to allow the selective separation and retain the VFAs within the liquid stream, while rejecting the suspended solids. To apply this technology effectively, pretreatment of the fermented effluent is necessary since membrane fouling can be the major operational deficiency limiting the wider adoption of the process for the S/L separation.

This work studied the integration of the sewage sludge fermentation–membrane separation (SF–MS) process in order to recover SCFAs from sewage sludge and subsequently use them for nutrient bioremoval in a WWTP. The use of a suitable alkaline silicate mineral (wollastonite) was studied to enhance the performance of the fermentation process and the subsequent S/L separation stage. Finally, the feasibility of using the SFL as carbon source for enhanced BNR was assessed.

Section snippets

Source of sewage sludge and reasons for wollastonite selection

The sewage sludge used for the fermentation experiments was collected from the municipal WWTP of Carbonera (Veneto Region, Italy). It consisted of mixed primary and secondary sludge. The use of wollastonite (Progind, Italy) to adjust the pH in the fermenter was tested as this could also lead to several advantages. SCFA production during sludge fermentation (SF) can provide the necessary acidity for dissolution of silicate minerals, resulting in the release of carbonate ion (CO32−) during the

Sewage sludge characteristics

The main physicochemical characteristics of the sewage sludge used in the pilot and bench scale fermentation reactors are the following: pH = 6.4 ± 0.3, TSS = 19,140 ± 151 mg/L, VSS = 15,645 ± 164 mg/L, COD = 18,221 ± 855 mg/L, sCOD = 274 ± 56 mg/L, TP = 291.1 ± 49, soluble phosphorus = 12.6 ± 2.7 mg P/L, TN = 729.3 ± 121 mg N/L and soluble nitrogen = 46.1 ± 13 mg N/L.

Effect of wollastonite concentration on sludge fermentation in bench scale

Fig. 2 shows the effect of different wollastonite concentrations (1–40 g/L) on the SCFA production (Fig. 2a) and on the SCFA composition (Fig. 2b) obtained for the bench scale

Conclusions

The long-term operation of the pilot scale alkaline sludge fermentation-membrane separation unit was examined. The use of 10 g/L wollastonite was beneficial as it maintained the pH above 7.0, increasing the SCFAs production, enhancing filterability and limiting the release of nutrients in the SFL. The sludge dewatering characteristics and the separation process were adversely affected from the use of caustic soda. The addition of wollastonite during fermentation was beneficial for filterability

Acknowledgements

The authors thank Alto Trevigiano Servizi Srl for funding and technical assistance. This work was co-funded by the Marie Curie FP7-PEOPLE-2012 project Low Environmental Footprint Biological Processes for Integrated Waste and Wastewater Treatment, LEF-BIOWASTE, (322333).

References (27)

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