Live Steam-Pretreatment and Anaerobic Digestion of Waste Activated Sludge

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  • Live Steam-Pretreatment and Anaerobic Digestionof Waste Activated Sludge

    Zhedong Gao, Cunsheng Zhang, Haijia Su,* and Tianwei Tan

    State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China.

    Received: November 25, 2012 Accepted in revised form: May 4, 2013

    Abstract

    Steam pretreatments mixing waste activated sludge (WAS) with live steam were applied to decompose WAS,and to enhance its anaerobic digestion performance. Results showed that the optimum pretreatment conditionwas 120C for 10 min. Chemical oxygen demand (COD) of the pretreated group was improved to 14,000 mg/L,18 times higher compared with the control group. The concentration of extracellular polymeric substances andsoluble protein increased to 1866 mg/L and 1495 mg/L, respectively. Soluble COD increased exponentially withincreasing specific energy; only a specific energy input of 271 kJ/kgTS was required. Analysis indicated that theparticle size of sludge was reduced and the distribution was more uniform after the pretreatment. Results fromthe subsequent batch anaerobic digestion indicated that higher stability of the system and biogas productionwere obtained in the pretreated group. Compared with raw sludge, biogas production of pretreated sludge wasimproved by 122%, corresponding to 600 mL/g of volatile solids. The present study mostly assesses the livesteam pretreatment of WAS and compares obtained results with previously examined thermal, chemical, orultrasound pretreatments. Results demonstrate that live steam pretreatment significantly enhances the biogasproduction while requiring limited energy input only.

    Key words: anaerobic digestion; biogas; live steam pretreatment; waste activated sludge

    Introduction

    Owing to the fast development of wastewater treat-ment plants, thousands of tons of waste activated sludge(WAS) are produced at a rate between 60 and 90 g per pop-ulation equivalent daily (Appels et al., 2008a). Usually, treat-ment and disposal of the residual sludge in traditionalapproaches were estimated to represent 2050% of the totaloperation cost of a wastewater treatment plant (Naddeo et al.,2009). However, landfill and soil application are still the maindisposal methods in many countries, causing severe envi-ronmental pollution and spreading pathogens (Chen et al.,2001; Liu et al., 2012). To deal with these problems, anaerobicdigestion (Appels et al., 2008a) is applied for sewage sludgetreatment, resulting in the reduction of residual sludgeby *25%, the production of biogas, and destruction of mostpathogens in the sludge, while limiting odor problems asso-ciated with uncontrolled putrescible matters.

    WAS is a complex matter which generally comprises ofbacteria cells, extracellular polymeric substances (EPS) andinorganic inert materials. All the physical-chemical parame-ters, including existence form (scattered or gathered), position

    (intracellular or exocellular), molecular structure (long orshort chain), chemical composition (such as polysaccharide,proteins, and DNA) and solubility characteristics could in-fluence the performance of anaerobic digestion of WAS. Toachieve an exhaustive disintegration of WAS, pretreatment isneeded (Nickel and Neis, 2007).

    Many pretreatment techniques have been reported to dis-integrate the sludge on lab-scale and pilot-scale plants overthe past decades. Some examples include ultrasonic treatment(Weemaes and Verstraete, 1998; Tiehm et al., 2001; Dewil et al.,2006a, 2006b; Nickel and Neis, 2007; Appels et al., 2008a,2008b; Pham et al., 2009; Erden et al., 2010), ozone oxidation(Weemaes and Verstraete, 1998; Weemaes et al., 2000; Neyenset al., 2002; Neyens et al., 2003b; Neyens and Baeyens, 2003a;Dewil et al., 2007; Erden et al., 2010), mechanical disintegration(Harrison, 1991; Kopp et al., 1997; Weemaes and Verstraete,1998; Nah et al., 2000; Lehne et al., 2001), acid and alkalinetreatment (Neyens and Baeyens, 2003b; Elsanousi et al., 2007),thermal treatment (Barlindhaug and degaard, 1996; Bar-jenbruch and Kopplow, 2003; Gavala et al., 2003), Fentonprocess (Kaynak and Filibeli, 2008; Erden and Filibeli, 2010),enzymatic hydrolysis (Thomas et al., 1993; Guellil et al., 2001;Ayol et al., 2008), or combinations of these techniques (Appelset al., 2008a; Chauzy et al., 2008). Thermal pretreatment hasbeen studied to improve the anaerobic digestibility and de-watering properties. Alkaline pretreatment has been also usedto solubilize various substrates, such as lignocellulosic

    *Corresponding author: State Key Laboratory of Chemical ResourceEngineering, Beijing University of Chemical Technology, Beijing100029, China. Phone: 8610-64452756; Fax: 8610-64414268; E-mail:suhaijia@126.com

    ENVIRONMENTAL ENGINEERING SCIENCEVolume 30, Number 9, 2013 Mary Ann Liebert, Inc.DOI: 10.1089/ees.2012.0480

    546

  • materials or WAS (Penaud et al., 1999). Chiu and colleagues(1997) showed that chemical oxygen demand (COD) solubili-zation, volatile solids (VS) reduction, and biogas productionwere enhanced when WAS was pretreated with NaOH. Ultra-sonic disintegration is a well-known method for rupturing mi-crobial cells to release intracellular material. It was reported thata specific energy level of 5005000 kJ/kgTS is effective for sludgedisintegration and this application led to an increasing methaneproduction in anaerobic digestion (Bougrier et al., 2006; Erdenet al., 2010). However, none of the above pretreatment methodscan meet the requirements of both easy operation and low cost.To solve this problem, the method of live steam pretreatmentwas introduced and fully examined in this article.

    Steam explosion is an economical and environmentallyfriendly process, which is extensively used in destroying thestructural components of biomass (e.g., crop straw) (Yu andChen, 2010). Steam explosion is typically initiated at a hightemperature of 160260C and a high pressure of 0.694.83 MPa. The state is maintained only a few seconds beforethe biomass being exposed to atmospheric pressure, resultingin the disrupting of extremely hard structures. However,WAS, which consists of 6070% organic substances (proteinand polysaccharide), is completely different from straw. Arelatively smooth heating pattern with much longer durationwould improve the disruption of sludge. Live steam pre-treatment was applied in this article with three differentconditions (100C at the pressure of 0.1 MPa; 120C at 0.2MPa; 140C at 0.36 MPa).

    To the best of our knowledge, enhancing biogas productionby adding live steam into WAS is a relatively novel andscarcely reported process (Chauzy et al., 2008). Accordingly, thepurpose of this work was to investigate the effects of differentconditions on the steam-pretreatment of WAS. The ammonia-nitrogen and pH were detected to evaluate the stability of theanaerobic digestion system. The soluble COD (SCOD), the ratioof EPS to soluble protein (SP), and the methane productionwere measured to confirm whether the live steam pretreatmentwas a better way to deal with sludge in comparison with thegeneral thermal, physical, or chemical methods.

    Materials and Methods

    Waste activated sludge

    WAS samples were obtained from the Shunyi municipalwastewater treatment plant, which serves part of Beijing witha handling capacity of about 160,000 population equivalent.After settling in the secondary clarifier, the WAS was thick-ened using a thickening table to reach a moisture content of8590%. To facilitate the storage of WAS, the concentratedsludge was air dried for 20 days at ambient temperature,collected in sealed plastics bags and stored at - 20C for laterexperiments. The experiments proved that there was no dif-ference between a suspension of 10% (w/w) dried sludge andfresh 10% (w/w) WAS in digestion yields. General charac-teristics of the samples are shown in Table 1, with averagevalues and deviations of two repeat measurements included.

    Steam pretreatment

    For pretreatment experiments, the WAS samples wereprepared at a moisture content of 90 wt%, with equivalentsolution total volume of about 150 mL for each group.

    A laboratory-scale steam generator was used for the livesteam-pretreatment. The pretreatment temperature was con-trolled at 100C, 120C and 140C, with a saturated steampressure of about 0.1, 0.2 and 0.36 MPa, respectively. Thepretreatment time was performed from 5 to 40 min (5, 10, 20,30, and 40 min). The warming and cooling process was quiteshort. The heat loss was < 5% for each experimental group,which could be ignored. All experiments were conducted induplicate. The errors of duplicate measurements were < 5%for all parameters; hence, average values are given. In thisstudy, the degree of sludge disintegration was calculatedby the difference of the COD/EPS ratio between the steam-pretreatment and the methods reported elsewhere. Electronmicroscopy was used for imaging the disintegration of sludgeflocs.

    Anaerobic batch digestion tests

    Experiments were carried out in bench scale anaerobic re-actors of 1.0 L (total volume), immersed in a thermostaticwater bath at 38 1C. The reactors were fed with raw an-aerobic inoculum at the ratio of 5% (w/w) and various WASsamples: untreated, common thermally-treated, and livesteam-treated. The available volume of the digestion liquidwas 0.8 L. The dosage of sludge was 2.4 g VS for each group.For the common thermal-pretreatment group, the sludgesolution was boiled at 100C for 10 min. For the steam-pretreatment group, the sludge solution was heated by thesteam generator at 100 and 120C for the required time. Thesamples were cooled down to room temperature before beingadded to the anaerobic digester.

    Initial pH was 7.27.3 and the digestion period was about10 days. The produced biogas was collected in a gas-collectingbag. The biogas yield was calculated by drainage methodthrough a peristaltic pump. When the pressure in the bagsreached 1000 mbar, a sample of gas was removed and thepressure released. Biogas and methane production weremonitored until the biogas production of each batch experi-ment reached stable levels and the production rate ap-proached to zero. The pH and biogas yield were obtaineddaily. The digestion liquid was sampled at regular time in-tervals (every 2 days) until the digestion was ultimately ter-minated.

    Analytical methods

    Chemical and instrumental analysis. The total solids (TS),VS, and COD were assayed according to conventionalmethods (ANSI) (Bougrier et al., 2006; Yu and Chen, 2010).

    Table 1. Characteristics of Waste Activated Sludge

    Parameter Unit Value

    Total solids % (w/w) 15.96 0.20Volatile solids % (w/w) 8.60 0.78Crude protein % (w/w), dry basis 16.23 0.11Crude polysaccharide % (w/w), dry basis 3.26 0.06Total carbon % (w/w), dry basis 63.10 0.50Total nitrogen % (w/w), dry basis 2.81 0.01Total COD g/L 23.30 0.10Soluble COD g/L 6.50 0.10

    COD, chemical oxygen demand.

    LIVE STEAM-PRETREATMENT ON WASTE ACTIVATED SLUDGE 547

  • For the EPS content, the samples were passed through0.25 lm pore size membrane filters, which were suitable toseparate protein and carbohydrates. The protein content wasdetermined by the Coomassie Brilliant Blue G-250 method(Bradford, 1976). The carbohydrate content was determinedusing a modified dinitrosalicylic acid (DNS) method, withglucose as the standard sample. The absorbance of the stan-dard sample and the ordinary samples was determined byspectrophotometer at 540 nm.

    The ammonia-nitrogen content was determined by amodified Nessler colorimetric method. The samples wereprocessed by adding sodium potassium tartrate tetrahydrateto screen main ions in the solution, such as Ca2 + and Mg2 + .All batches were performed in duplicate and the averagevalues are reported.

    Compositions and concentrations of gas samples (CH4/CO2/N2) were analyzed by gas chromatography (GC-2014C,Shimadzu), equipped with a thermal conductivity detector(TCD) and a TDX-01 packed (3 mm inside diameter) stainlesssteel column (2 m 3 mm). The temperatures of the column,injector, and the TCD detector were 160C, 160C and 180C,respectively. Argon was used as the carrier gas at a flow rateof 30 mL/min.

    The sludge size analysis of untreated and steam-treatedWAS was performed by scanning electron microscope (SEM).Average sizes were estimated from 10 images randomly takenon at least triplicate samples. Images were processed withPhotoshop software.

    Biogas composition and yield. The GC was calibrated byinjecting a reference gas mixture containing CO2, N2, CH4,and H2 with component concentrations of 19.9%, 39.2%,39.9% and 1.01% (v/v), respectively. CH4 production wascalculated using the following equation (Gurung et al.,2012):

    VR, i VR, i 1 CR, i(VT, i VT, i 1)VH(CR, i CR, i 1 (1)

    where VR,i and VR,i - 1 are the cumulative CH4 gas volumes atthe current (i) and previous (i - 1) time intervals, respectively,VT,i and VT,i - 1 are the total biogas volumes in the currentand previous time intervals, CR,i and CR,i - 1 are the fraction ofCH4 in the current and previous intervals in the headspace ofthe digestion bottle measured using GC, and VH is the totalvolume of headspace in the reactor (150 mL).

    Cumulative biogas production curves were obtained overthe course of the batch experiment and analyzed using themodified Gompertz equation.

    Energy consumption analysis

    Similar to the method used for ultrasound pretreatment,the specific energy (SE) was introduced as a reference of en-ergy consumption in live steam-pretreatment. The SE (in kJ/kg TS) was calculated using the following equation:

    SEH M t (2)

    where H was the enthalpy of saturated steam in kJ/kgSteam,M was the steam dosage in kgSteam/(kg TS$h), and t was thesteam-pretreatment time in h. Due to thermal losses in thesystem, the actual steam energy imparted to the sludgeis lower than the amount of energy applied by the steam

    device, and the heat loss was *5% (Kobus and Kusinska,2008).

    The actual energy transferred to the sludge with commonthermal pretreatment has been calculated using the thermalmethod recommended initially by Gonzalez et al. (1999). Theactual energy requirements for heating sludge were calcu-lated based on the following equation (Ros and Zupancic,2003):

    Qs qsVsCp(Tfinal Tinitial) (3)

    where Qs is the quantity of heat required to heat the sludge inkJ, qs is the density of sludge in kg/m

    3, Vs is the volume of thetreated sludge in m3, Cp is the specific heat of sludge in kJ/(kg$C) [the value of water and dry sludge were 4.18 and 0.84kJ/(kg$C), respectively], Tinitial is the initial temperature ofsludge in C, and Tfinal is the final temperature of sludge in C.The initial temperature of sludge was 25C and the heat lossduring thermal pretreatment equipment was 5% (Dhar et al.,2011a, 2011b). The energy consumption of live steam and solethermal pretreatment was calculated and compared to theresults of recent literature (Neyens et al., 2003b; Bougrier et al.,2006; Appels et al., 2008a).

    Results and Discussion

    Effects of pretreatment on the disintegration of WAS

    The degree of WAS disintegration is a significant p...

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