Direct discharge of slaughterhouse wastewater causes serious environmental pollution due to its high chemical oxygen demand (COD), total suspended solids (TSS) and biochemical oxygen demand. In this study, an ultrasonic-assisted membrane anaerobic system was used as a novel method for treating slaughterhouse wastewater. Six steady states were achieved, using concentrations of 7,800–13,620 mg/l for mixed liquor suspended solids and 5,359–11,424 mg/l for mixed liquor volatile suspended solids (MLVSS). Kinetic equations were used to describe the kinetics of treatment at organic loading rates of 3–11 kg COD/m3/d. The removal efficiency of COD was 94.8–96.5% with hydraulic retention times of 308.6–8.7 days. The growth yield coefficient was found to be 0.52 g VSS/g. COD was 0.21 d−1 and methane gas production rate was 0.24–0.56 l/g COD/d. Steady-state influent COD concentrations increased from 8,000 mg/l in the first steady state to 25,400 mg/l in the sixth steady state. The minimum solids retention time, θcmin obtained from the three kinetic models was 6–14.4 days. The k values were 0.35–0.519 g COD/g VSS.d and μmax values were between 0.26 and 0.379 d−1. The solids retention time decreased from 600 to 14.3 days. The complete treatment reduced the COD content and its removal efficiency reached 94.8%.

ABBREVIATIONS AND NOMENCLATURE

  • COD

    chemical oxygen demand (mg/l)

  • OLR

    organic loading rate (kg/m3/d)

  • TSS

    total suspended solid (mg/l)

  • MLSS

    mixed liquor suspended solids (mg/l)

  • HRT

    hydraulic retention time (day)

  • SSUR

    specific substrate utilisation rate (kg COD/kg VSS/d)

  • UMAS

    ultrasonic membrane anaerobic system

  • VSS

    volatile suspended solids (mg/l)

  • U

    specific substrate utilisation rate (SSUR) (g COD/G VSS/d)

  • S

    effluent substrate concentration (mg/l)

  • So

    influent substrate concentration (mg/l)

  • X

    micro-organism concentration (mg/l)

  • maximum specific growth rate (day−1)

  • K

    maximum substrate utilisation rate (COD/g/VSS.day)

  • half velocity coefficient (mg COD/l)

  • B

    specific microorganism decay rate (day−1)

  • Y

    growth yield coefficient (gm VSS/gm COD)

  • T

    time

INTRODUCTION

Wastewater from a slaughterhouse arises from different steps of the slaughtering process, such as washing of animals, bleeding out, skinning, cleaning of animal bodies and cleaning of rooms; thus, the main pollutant in slaughterhouse effluents is organic matter. The effluents carry blood, particles of skin and meat, excrement and other pollutants. This wastewater is very harmful to the environment. Effluent discharge from slaughterhouses has caused the deoxygenation of rivers (Quinn & McFarlane 1989) and the contamination of groundwater (Dinopoulu et al. 1988; Sangodoyin & Agbawhe 1992). The pollution potential from meat-processing and slaughterhouse plants has been estimated at over 1 million population equivalents in the Netherlands (Sayed 2005), and 3 million in France. Blood, one of the major dissolved pollutants in slaughterhouse wastewater, has a chemical oxygen demand (COD) of 375,000 mg/l (Tritt & Schuchardt 1992). Slaughterhouse wastewater also contains high concentrations of suspended solids, including pieces of fat, grease, hair, feathers, flesh, manure, grit and undigested feed. These insoluble and slowly biodegradable suspended solids represented 50% of the pollution charge in screened (1 mm) slaughterhouse wastewater, while another 25% originated from colloidal solids (Sayed et al. 1988). Typical characteristics of wastewater from slaughterhouse are given in Table 1.

Table 1

Characteristics of the wastewater from slaughterhouse (Quinn & McFarlane 1989)

Parameter Concentration (g/l) 
pH 6.8–7.8 
COD 5.2–11.4 
TSS 0.57–1.69 
Phosphorus 0.007–0.0283 
Ammoniacal nitrogen 0.019–0.074 
Protein 3.25–7.86 
Parameter Concentration (g/l) 
pH 6.8–7.8 
COD 5.2–11.4 
TSS 0.57–1.69 
Phosphorus 0.007–0.0283 
Ammoniacal nitrogen 0.019–0.074 
Protein 3.25–7.86 

Slaughterhouse wastewater quality depends on a number of factors, namely:

  1. Blood capture: the efficiency in blood retention during animal bleeding is considered to be the most important measure for reducing biological oxygen demand (BOD) (Tritt & Schuchardt 1992).

  2. Water usage: water economy usually translates into increased pollutant concentration, although total BOD mass will remain constant.

  3. Type of animal slaughtered: BOD is higher in wastewater from beef than hog slaughterhouses (Tritt & Schuchardt 1992).

  4. Amount of rendering or meat processing activities: plants that only slaughter animals produce a stronger wastewater than those also involved in rendering or meat processing activities.

Anaerobic ponds are commonly used to achieve a high degree of BOD reduction in slaughterhouse wastewater. However, this method suffers from the disadvantage of odour generation from the ponds thus making the development of alternate designs essential. Anaerobic contact, up-flow anaerobic sludge blankets and anaerobic filter reactors have been tried for slaughterhouse wastes. All these have a higher organic loading rate (OLR) ranging from 5 to 40 kg COD/m3/day (Ruiz et al. 1997). The high rate anaerobic treatment systems, such as upflow anaerobic sludge blanket and fixed bed reactors, are less popular for slaughterhouse wastes due to the presence of high fat, oil and suspended matter in the influent. This affects the performance and efficiency of the treatment systems. Also, because of relatively low BOD, high rate systems which function better for higher BOD concentrations are not appropriate. Table 2 summarises the performance data of digesters used for the treatment of slaughterhouse wastewater. In recent years, considerable attention has been paid towards the development of reactors for anaerobic treatment of wastes leading to the conversion of organic molecules into biogas. These reactors, known as second generation reactors or high rate digesters, can handle wastes at a high OLR of 24 kg COD/m3/day and high up-flow velocity of 2–3 m/h at a low hydraulic retention time (HRT) (Ruiz et al. 1997). However, the treatment efficiencies of these reactors are sensitive to parameters like wastewater composition, especially the concentration of various ions (Johns 1995; Ruiz et al. 1997) and presence of toxic compounds, such as phenol (Lettinga 1995). The temperature and pH are also known to affect the performance of the reactor by affecting the degree of acidification of the effluent and the product formation (Zhang & Maekawa 1996). Table 2 shows some treatment systems for slaughterhouse wastes, and Table 3 shows mathematical expressions for specific substrate utilisation rate for three kinetic models: the Monod, Contois and Chen & Hashimoto models.

Table 2

Treatment systems for slaughterhouse wastes (Sangodoyin & Agbawhe 1992)

Reactor Capacity (m3OLR (kg COD/m3/day) Reduction (%) 
UASB (granular) 33 11 85 
UASB (flocculated) 10 80–89 
Anaerobic filter 21 2.3 85 
Anaerobic contact 11–20 92.6 
Reactor Capacity (m3OLR (kg COD/m3/day) Reduction (%) 
UASB (granular) 33 11 85 
UASB (flocculated) 10 80–89 
Anaerobic filter 21 2.3 85 
Anaerobic contact 11–20 92.6 
Table 3

Mathematical expressions of specific substrate utilisation rates for known kinetic models

Kinetic model Equation (1) Equation (2) 
Monod (1949)    
Contois (1959)    
Chen & Hashimoto (1980)    
Kinetic model Equation (1) Equation (2) 
Monod (1949)    
Contois (1959)    
Chen & Hashimoto (1980)    

An improvement in the efficiency of anaerobic digestion can be brought about by either modifying the existing digester design or by incorporating appropriate advanced techniques. Thus, a plug flow reactor or up-flow staged sludge bed reactor is found to be superior to the conventional processes, due to low concentrations of volatile fatty acids in the effluent, a high degree of sludge retention and stable reactor performance (Mudrak & Kunst 1986). Another common problem encountered in the industrial anaerobic plants is biomass washout. This can be addressed, for instance, by the use of membranes coupled with an anaerobic digester for biomass retention (Fang & Chan 1997). This paper aims to introduce a new technique, the ultrasonic membrane anaerobic system (UMAS) for slaughterhouse wastewater treatment. This system overcomes the membrane fouling problems.

MATERIALS AND METHODS

Raw slaughterhouse wastewater was treated by UMAS in a laboratory digester with an effective 200-l volume. Figures 1 and 2 present a schematic representation of the UMAS which consists of a cross-flow ultra-filtration membrane apparatus, a centrifugal pump and an anaerobic reactor. Six multi frequency ultrasonic transducers, operated at 25 KHz, are bonded to two sides of the tank chamber and connected to a Crest Genesis Generator (250 W, 25 KHz; Crest Ultrasonics, Trenton, NJ, USA). The principle of ultrasonic treatment relies on cavitation to disintegrate cell walls. High density intensity ultrasound enhances the disintegration of particulate matter, as shown by a reduction in particle size and increase of the soluble matter fraction (Wang et al. 2005; Benabdallah et al. 2006).

Figure 1

Experimental set-up.

Figure 1

Experimental set-up.

Figure 2

Experimental schematic for UMAS.

Figure 2

Experimental schematic for UMAS.

The ultra-filtration membrane module had a molecular weight cut-off of 200,000, a tube diameter of 1.25 cm and an average pore size of 0.1 μm. There were four tubes, each 30 cm long, and the total effective area of the four membranes was 0.048 m2. The maximum operating pressure on the membrane was 55 bars at 70 °C, and the pH ranged from 2 to 12. The reactor was composed of a heavy duty reactor with an inner diameter of 25 cm and height of 250 cm. The operating pressure in the UMAS was maintained between 2 and 4 bars by manipulating the gate valve in the retentate line after the cross-flow ultra-filtration membrane unit.

Slaughterhouse wastewater

Raw slaughterhouse wastewater samples were collected from a slaughterhouse in Kuantan, Malaysia. The wastewater was stored in a cold room at 4 °C prior to use. Samples analysed for COD, total suspended solids (TSS), pH, volatile suspended solids (VSS), substrate utilisation rate and specific substrate utilisation rate (SSUR).

Analytical methods

Biogas volume was daily measured with water displacement, using a 20-litre water displacement bottle, and methane content was analysed by a J-Tube analyser and a gas chromatograph (GC 2011, Shimadzu) equipped with a thermal conductivity detector and a 2 m × 3 mm stainless-steel column packed with Porapak Q (80/100 mesh). TSS, VSS, volatile fatty acids and alkalinity were determined according to the Standard Methods (APHA 2005). The COD was measured using a Hach colorimetric digestion method (Method # 8000, Hach Company, Loveland, CO, USA).

Bioreactor operation

The UMAS performance was evaluated under six steady states with influent COD concentrations ranging from 8,000 to 25,400 mg/l and OLR between 3.0 and 11 kg COD/m3/d. In this study, the system was considered to have achieved steady state when the operating and control parameters were within ±10% of the average value. The biogas produced was mainly carbon dioxide and methane, so sodium hydroxide solution was added to absorb the carbon dioxide; the remaining gas was methane. Table 5 depicts results of the application of three known substrate utilisation models.

RESULTS AND DISCUSSION

Semi-continuous UMAS performance

Table 4 summarises UMAS performance at six steady states, which were established at different HRTs and influent COD concentrations. The kinetic coefficients of the selected models were derived from Equation (2) in Table 3 by using a linear relationship; the coefficients are summarised in Table 5. At steady-state conditions with influent COD concentrations of 8,000–25,400 mg/l, UMAS performed well and the pH in the reactor remained within the optimal working range for anaerobic digesters (6.7–7.8). At the first steady state, the mixed liquor suspended solid (MLSS) concentration was about 7,800 mg/l and the MLVSS concentration was 5,329 mg/l, equivalent to 68.7% of the MLSS. This low result can be attributed to the high suspended solids contents in the slaughterhouse wastewater. At the sixth steady state, however, the VSS fraction in the reactor increased to 88% of the MLSS. This indicates that the long solids retention time of UMAS facilitated the decomposition of the suspended solids and their subsequent conversion to methane; this conclusion is in agreement with previous studies (Nagano et al. 1992; Abdurahman et al. 2011). The highest influent COD was recorded at the sixth steady state (25,400 mg/l) and corresponded to an OLR of 11 kg COD/m3/d. At this OLR, the UMAS achieved 96.7% COD removal and an effluent COD of 3000 mg/l. This value is better than those reported in other studies on anaerobic slaughterhouse wastewater digestion (Ng et al. 1985; Borja-Padilla & Banks 1993; Van Lier et al. 1994). The three kinetic models demonstrated a good relationship (R2 > 99%) for the membrane anaerobic system treating slaughterhouse wastewater, as shown in Figures 345. The Contois and Chen & Hashimoto models performed better, implying that digester performance should consider OLRs. These two models suggested that the predicted permeate COD concentration (S) is a function of influent COD concentration (So). In the Monod model, however, S is independent of So. The excellent fit of these three models (R2 > 97.8%) in this study suggests that the UMAS process is capable of handling sustained organic loads between 3 and 11 kg m3/d.

Table 4

Summary of steady state results

Steady state 
COD feed, mg/L 8,000 10,700 15,400 18,700 20,000 25,400 
COD permeate, mg/L 280 428 662 860 920 1,321 
Gas production, (L/d) 190.5 220 260 320 360 373 
Total gas yield, L/g COD/d 0.32 0.32 0.48 0.54 0.62 0.68 
% methane 74 70.5 68.6 67.6 64.2 61.8 
Methane yield, l/g COD/d 0.29 0.32 0.50 0.54 0.56 0.59 
MLSS, mg/L 7,800 8,740 10,080 11,280 12,546 13,620 
MLVSS, mg/L 5,359 7,428 8,840 10,340 11,120 11,424 
% VSS 68.71 84.99 87.70 91.67 88.63 88.87 
HRT, d 308.6 60.3 13.9 10.86 9.64 8.7 
SRT, d 580 298 127 26.8 13.44 11.8 
OLR, kg COD/m3/d 3.0 5.0 7.0 8.2 9.0 11 
SSUR, kg COD/kg VSS/d 0.164 0.195 0.252 0.263 0.294 0.314 
SUR, kg COD/m3/d 0.023 0.724 2.225 4.576 5.685 7.347 
Percent COD removal (UMAS) 96.5 96.0 95.7 95.4 95.4 94.8 
Steady state 
COD feed, mg/L 8,000 10,700 15,400 18,700 20,000 25,400 
COD permeate, mg/L 280 428 662 860 920 1,321 
Gas production, (L/d) 190.5 220 260 320 360 373 
Total gas yield, L/g COD/d 0.32 0.32 0.48 0.54 0.62 0.68 
% methane 74 70.5 68.6 67.6 64.2 61.8 
Methane yield, l/g COD/d 0.29 0.32 0.50 0.54 0.56 0.59 
MLSS, mg/L 7,800 8,740 10,080 11,280 12,546 13,620 
MLVSS, mg/L 5,359 7,428 8,840 10,340 11,120 11,424 
% VSS 68.71 84.99 87.70 91.67 88.63 88.87 
HRT, d 308.6 60.3 13.9 10.86 9.64 8.7 
SRT, d 580 298 127 26.8 13.44 11.8 
OLR, kg COD/m3/d 3.0 5.0 7.0 8.2 9.0 11 
SSUR, kg COD/kg VSS/d 0.164 0.195 0.252 0.263 0.294 0.314 
SUR, kg COD/m3/d 0.023 0.724 2.225 4.576 5.685 7.347 
Percent COD removal (UMAS) 96.5 96.0 95.7 95.4 95.4 94.8 

MLVSS, mixed liquor volatile suspended solid; SRT, solids retention time; SUR, substrate utilisation rate.

Figure 3

The Monod model.

Figure 3

The Monod model.

Figure 4

The Contois model.

Figure 4

The Contois model.

Figure 5

The Chen and Hashimoto model.

Figure 5

The Chen and Hashimoto model.

Figure 6 shows the percentages of COD removed by UMAS at various HRTs. COD removal efficiency increased as HRT increased from 8.7 to 308.6 days and was in the range of 94.8–96.5%. This result was higher than the 85% COD removal observed for slaughterhouse wastewater treatment using anaerobic fluidised bed reactors (Idris & Al-Mamun 1998) and the 91.7–94.2% removal observed for slaughterhouse wastewater treatment using MAS (Fakhru'l-Razi 1994), and the 93.6–97.5% removal observed for palm oil mill effluent (POME) treatment using MAS (Abdurahman et al. 2011). The COD removal efficiency did not differ significantly between HRTs of 480.3 days (98.5%) and 20.3 days (98.0%). On the other hand, the COD removal efficiency was reduced with shorter HRTs; at HRT of 5.40 days, COD was reduced to 96.7%. As shown in Table 4, this was largely a result of the washout phase of the reactor because the biomass concentration increased in the system. This may attributed due to the fact that at low HRT with high OLR, the organic matter was degraded to volatile fatty acids. The HRTs were mainly influenced by the ultra-filtration membrane influx rates which directly determined the volume of influent that can be fed to the reactor.

Figure 6

COD removal efficiency of UMAS under steady-state conditions with various hydraulic retention times.

Figure 6

COD removal efficiency of UMAS under steady-state conditions with various hydraulic retention times.

Determination of bio-kinetic coefficients

Experimental data for the six steady-state conditions in Table 4 were analysed; kinetic coefficients were evaluated and are summarised in Table 5. SURs and specific substrate SSURs were plotted against OLRs and HRTs. Figure 7 shows the SSUR values for COD at steady-state conditions with HRTs between 8.7 and 308.6 days. SSURs for COD generally increased proportionally HRT declined, which indicated that the bacterial population in the UMAS multiplied (Abdullah et al. 2005). The bio-kinetic coefficients of growth yield (Y) and specific micro-organic decay rate (b); and the K values were calculated from the slope and intercept as shown in Figures 8 and 9. Maximum specific biomass growth rates (μmax) were in the range between 0.291 and 0.377 d−1. All of the kinetic coefficients that were calculated from the three models are summarised in Table 5. The small values of μmax are suggestive of relatively high amounts of biomass in the UMAS (Zinatizadeh et al. 2006). According to Grady & Lim (1980) and Masse & Masse (2005), the values of parameters μmax and K are highly dependent on both the organism and the substrate employed. If a given species of organism is grown on several substrates under fixed environmental conditions, the observed values of μmax and K will depend on the substrates.

Table 5

Results of the application of three known substrate utilisation models

Model Equation R2 (%) 
Monod (1949)   98.9 
Contois (1959)   97.8 
Chen & Hashimoto (1980)   98.7 
Model Equation R2 (%) 
Monod (1949)   98.9 
Contois (1959)   97.8 
Chen & Hashimoto (1980)   98.7 
Figure 7

Specific substrate utilization rate for COD under steady-state conditions with various hydraulic retention times.

Figure 7

Specific substrate utilization rate for COD under steady-state conditions with various hydraulic retention times.

Figure 8

Determination of the growth yield, Y and the specific biomass decay rate, b.

Figure 8

Determination of the growth yield, Y and the specific biomass decay rate, b.

Figure 9

Determination of the maximum specific substrate utilization and the saturation constant, K.

Figure 9

Determination of the maximum specific substrate utilization and the saturation constant, K.

Gas production and composition

Many factors must be adequately controlled to ensure the performance of anaerobic digesters and prevent failure. For slaughterhouse wastewater treatment, these factors include pH, mixing, operating temperature, nutrient availability and OLRs into the digester. In this study, the microbial community in the anaerobic digester was sensitive to pH changes. Therefore, the pH was maintained in an optimum range (6.8–7) to minimise the effects on methanogens that might affect biogas production. Because methanogenesis is strongly affected by pH, methanogenic activity will decrease when the pH in the digester deviates from the optimum value. Mixing provides good contact between microbes and substrates, reduces the resistance to mass transfer, minimises the build-up of inhibitory intermediates and stabilises environmental conditions. This study adopted the mechanical mixing and biogas recirculation. Figure 10 shows the gas production rate and the methane content of the biogas. The methane content generally declined with increasing OLRs. This is possibly due to inhibition by polyphenols, as has been suggested previously (Rozzi et al. 1986; Boari & Mancini 1989).

Figure 10

Gas production and methane content.

Figure 10

Gas production and methane content.

Methane gas contents ranged from 61.8 to 74% and the methane yield ranged from 0.29 to 0.59 g COD/d. Biogas production increased with increasing OLRs from 0.32 l/g COD/d at 3 kg COD/m3/d to 0.68 l/g COD/d at 11 kg COD/m3/d. The decline in methane gas content may be attributed to the higher OLR, which favours the growth of acid-forming bacteria over methanogenic bacteria (Ng et al. 1985; Borja-Padilla & Banks 1993; Ross & Strogwald 1994). Thus the methane conversion process was adversely affected with reducing methane content and this has led to the formation of carbon dioxide at a higher rate. The gas production showed an increase from 190.5 to 373 l per day during the study. In this scenario, the declining methane content can be attributed to the higher OLRs which favour a higher growth rate of acidogenic bacteria over the methanogenic bacteria.

CONCLUSIONS

UMAS seemed to be adequate for the biological treatment of undiluted slaughterhouse wastewater, since reactor volumes are needed which are considerably smaller than the volumes required by the conventional digester. UMAS was found to be an improvement and a successful biological treatment system that achieved high COD removal efficiency in a short period of time (no membrane fouling following introduction of ultrasonic treatment). The overall substrate removal efficiency was very high – about 96.5%. The gas production and methane concentration in the gas were satisfactory and, therefore, could be considered as an additional energy source for the use in the slaughterhouse. Preliminary data on anaerobic digestion at 30°C in UMAS showed that the proposed technology has good potential to substantially reduce the pollution load of slaughterhouse wastewater. UMAS was efficient in retaining the biomass. The UMAS process will recover a significant quantity of energy (methane 74%) that could be used to heat or produce hot water at the slaughterhouse wastewater plant.

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