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The Potentials of ultrasonic membrane anaerobic system (UMAS) in treating slaughterhouse wastewater

Written By

Abdurahman Nour

Submitted: May 5th, 2014 Reviewed: August 26th, 2014

DOI: 10.5772/59018

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1. 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, cleaning of rooms, etc. the main pollutant in slaughterhouse effluents is organic matter. The contributions of organic load to these effluents are blood, particles of skin and meat, excrements and other pollutants. Slaughterhouse wastewater is very harmful to the environment. Effluent discharge from slaughterhouses has caused the deoxygenation of rivers [Quinn et al., 1989] and the contamination of groundwater [Sangodoyin et al., 1992]. The pollution potential of meat-processing and slaughterhouse plants has been estimated at over 1 million population equivalent 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 375000 mg/l [Tritt el al., 1992]. Slaughterhouse wastewater also contains high concentrations of suspended solids(SS), including pieces of fat, grease, hair, feathers, flesh, manure, grit, and undigested feed. These insoluble and slowly biodegradable SS 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.

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

Table 1.

Characteristics of the wastewater from the slaughterhouses [Quinn et al., 1989]

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 et al., 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 et al., 1992]

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

Most slaughterhouse wastewater quality data have been generated in Europe [Sayed et al., 2005; Tritt et al., 1992 and Sayed et al., 1988].

Anaerobic ponds are commonly used to achieve a high degree of BOD reduction in slaughterhouse wastewater. However, this suffers from the disadvantage of odour generation from the ponds thus making the development of alternate designs very essential. Anaerobic contact, up-flow anaerobic sludge blanket, 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 UASB and fixed bed reactors are less popular for slaughterhouse wastes due to the presence of high fat oil and suspended matters 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 appreciate. Table 2 summarizes 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 organic loading rate of 24 kg COD/m3/day and high up-flow velocity of 2-3 m/h at a low hydraulic retention time [Ruiz et al., 1997]. However, the treatment efficiencies of these reactors are sensitive to parameters like wastewater composition, especially the concentration of various ions [Ruiz et al., 1997; Johns, 1995] 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 et al., 1996]. Table 2 shows some treatment systems for slaughterhouse wastes, while Table 3 shows mathematical expressions for specifics substrate utilization rate for three kinetic models.

ReactorCapacity (m3)OLR (kg COD/m3/day)Reduction (%)
UASB (granular)33118.5
UASB (flocculated)10580-89
Anaerobic filter212.385
Anaerobic contact11.120392.6

Table 2.

Treatment systems for slaughterhouse wastes [Sangodoyin et al., 1992]

Kinetic ModelEquation 1Equation 2
MonodU=kSks+S1U=KsK(1S)+1k(1949)
ContoisU=Umax×SY(B×X+S)1U=a×Xμmax×S+Y(1+a)μmax
Chen & HashimotoU=μmax×SYKSo+(1K)SY(1959)
1U=YKSoμmaxS+Y(1K)μmax(1980)

Table 3.

Mathematical expressions of specifics substrate utilization rates for known kinetic models

An improvement in the efficiency of anaerobic digestion can be brought about by either suitably modifying the existing digester design or by incorporating appropriate advanced techniques. Thus, a plug flow reactor or USSB reactor is found to be superior to the conventional processes due to low concentrations of VFA in the effluent, a high degree of sludge retention and stable reactor performance [Mudrak et al., 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 the anaerobic digester for biomass retention [Fang et al., 1997]. This paper introduces a new technique, which Ultrasonicated membrane anaerobic system (UMAS) for slaughterhouse wastewater treatment. This system, UMAS avoid and solve the membrane fouling problems.

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2. Slaughterhouse wastewater treatment: A review

2.1. Sewer discharge

Sewer discharge of wastewater without preliminary treatment is mostly used by smaller slaughterhouses located close to municipal treatment plants. Domestic wastewater, generally much lower in BOD and inorganic nutrient concentration, dilutes the slaughterhouse wastewater and makes it more amenable to biological treatment. The main disadvantage of sewer discharge is the surcharge imposed by municipalities to treat the wastewater. In addition, few municipal treatment plants will accept large quantities of untreated slaughterhouse wastewater.

2.2. Land application

Land application of slaughterhouse wastewater by spray irrigation has been mainly used in USA [Asselin et al., 2008]. The advantages of the system are its simplicity and low cost. The disadvantages include possible surface and ground water contamination, odour problems, greenhouse gas emission, and soil pore clogging from excessive fat loads. Application on constructed wetlands could also be used as a polishing treatment for biologically treated wastewater [John 1995]. Land application, however, is not practical in subfreezing temperatures, and in most parts of Canada large volumes of wastewater would have to be stored during the winter months.

2.3. Physico-chemical treatments

Grit chambers, screens, settling tanks, and dissolved air flotation (DAF) units are widely used for the removal of SS, colloidals, and fats from slaughterhouse wastewater. In DAF units, air bubbles injected at the bottom of the tank transport light solids and hydrophobic material, such as fat and grease, to the surface where scum is periodically skimmed off. [Masse et al., 2005] surveyed wastewater treatment in over 200 meat packing plants in the USA and concluded that, compared to aerobic and anaerobic systems, air flotation was the least efficient treatment in terms of dollars per weight of BOD removed.

Blood coagulants (e.g. aluminium sulphate and ferric chloride) and or flocculents (polymers) are sometimes added to the wastewater in the DAF unit to increase protein flocculation and precipitation as well as fat flotation. Chemical DAF units can achieve COD reduction ranging from 32 to 90%, and are capable of removing large amounts of nutrients [John 1995]. However, operational problems have been reported, and the system produces large volumes of putrefactive and bulky sludge that requires special handling and further treatment [John 1995].

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3. Materials and methods

With the increasing energy prices and the drive to reduce CO2 emissions, universities and industries are challenged to find new technologies in order to reduce energy consumption, to meet legal requirements on emissions, and for cost reduction and increased quality. The 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 (BOD). Traditional ways for slaughterhouse wastewater treatment have both economic and environmental disadvantages. In this study, ultrasonic assisted-membrane anaerobic system (UMAS) was used as an alternative, cost effective method for treating slaughterhouse wastewater (to avoid membrane fouling).

Raw slaughterhouse wastewater was treated by UMAS in a laboratory digester with an effective 200-litre volume. Figs. 1-2 presents a schematic representation of the ultrasonic-membrane anaerobic system (UMAS) which consists of a cross flow ultra-filtration membrane (CUF) apparatus, a centrifugal pump, and an anaerobic reactor. 25 KHz multi frequency ultrasonic transducers connected into the MAS system. The ultrasonic frequency is 25 KHz, with 6 units of permanent transducers and bonded to the two (2) sided of the tank chamber and connected to one (1) unit of 250 watts 25 KHz Crest’s Genesis Generator. The UF membrane module had a molecular weight cut-off (MWCO) of 200,000, a tube diameter of 1.25 cm and an average pore size of 0.1 µm. The length of each tube was 30 cm. The total effective area of the four membranes was 0.048 m². The maximum operating pressure on the membrane was 55 bars at 70 ºC, 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 a total height of 250 cm. The operating pressure in this study was maintained between 2 and 4 bars by manipulating the gate valve at the retentate line after the CUF unit.

Figure 1.

Experimental set-up

Figure 2.

Experimental schematic for UMAS

3.1. Slaughterhouse wastewater

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

3.2. Bioreactor operation

The ultrasonic membrane anaerobic system, UMAS Performance was evaluated under six steady-states, Table 4, with influent COD concentrations ranging from (8,000 to 25,400 mg/l) and organic loading rates (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. A 20-litre water displacement bottle was used to measure the daily gas volume. The produced biogas contained only CO2 and CH4, so the addition of sodium hydroxide solution (NaOH) to absorb CO2 effectively isolated methane gas (CH4).Table 5 depicts results of the application of three known substrate utilisation models

Steady State (SS)123456
COD feed, mg/L80001070015400187002000025400
COD permeate, mg/L2804286628609201321
Gas production (L/d)190.5220260320360373
Total gas yield, L/g COD/d0.210.320.480.540.620.68
% Methan7470.568.667.664.261.8
Ch4 yield, l/g COD/d0.290.320.500.540.560.59
MLSS, mg/L7800874010080112801254613620
MLVSS, mg/L535974288840103401112011424
% VSS68.7184.9987.7091.6788.6383.87
HRT, d308.660.313.910.869.648.7
SRT, d58029812726.813.4411.8
OLR, kg COD/m3/d3.05.07.08.29.011
SSUR, kg COD/kg VSS/d0.1640.1950.2520.2630.2940.314
SUR, kg COD/m3/d0.0230.7242.2254.5765.6857.347
Percent COD removal (UMAS)96.596.095.795.495.494.8

Table 4.

Summary of results (SS: steady state)

ModelEquationR2(%)
MonodU1=2025S1+3.61Ks=498K=0.350μMax=0.28498.9
ContoisU1=0.306XS1+2.78B=0.111uMax=0.344a=0.115μMax=0.377K=0.51997.8
Chen & HashimotoU1=0.0190SoS1+3.77K=0.006a=0.006μMax=0.291K=0.37498.7

Table 5.

Results of the application of three known substrate utilisation models

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4. Results and discussion

4.1. Semi-continuous Ultrasonic-Membrane Anaerobic System (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 Eq. (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 MLSS concentration was about 7,800 mg/l whereas the MLVSS concentration was 5,329 mg/l, equivalent to 68.71% 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 volatile suspended solids (VSS) fraction in the reactor increased to 88% of the MLSS. This indicates that the long SRT of UMAS facilitated the decomposition of the suspended solids and their subsequent conversion to methane (CH4); this conclusion supported by (Abdurahman et al., 2011) and (Nagano et al., 1992). The highest influent COD was recorded at the sixth steady-state (91,400 mg/l) and corresponded to an OLR of 9.5 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 (Borja et al., 1993; Ng et al., 1985). The three kinetic models demonstrated a good relationship (R2 > 99%) for the membrane anaerobic system treating slaughterhouse wastewater, as shown in Figs. 2-4. The Contois and Chen & Hashimoto models performed better, implying that digester performance should consider organic loading rates. These two models suggested that the predicted permeate COD concentration (S) is a function of influent COD concentration (So). In 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 0.5 and 9.5 kg m3/d.

Figure 3.

The Monod model

Figure 4.

The Contois model

Figure 5.

The Chen and Hashimoto model

Fig.5 shows the percentages of COD removed by UMAS at various HRTs. COD removal efficiency increased as HRT increased from 5.40 to 480.3 days and was in the range of 96.7 %-98.5 %. This result was higher than the 85 % COD removal observed for slaughterhouse wastewater treatment using anaerobic fluidised bed reactors (Idris et al., 1998) and the 91.7-94.2 % removal observed for slaughterhouse wastewater treatment using MAS (Fakhru’l-Razi et al., 1999), and the 93.6-97.5% removal observed for 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 shorter HRTs; at HRT of 5.40 days, COD was reduced to 96.7 %. As shown in Table 2, 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 (VFA). The HRTs were mainly influenced by the ultra-filtration, UF membrane influx-rates which directly determined the volume of influent (POME) that can be fed to the reactor.

Figure 6.

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

4.2. 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. Substrate utilisation rates (SUR); and specific substrate utilisation rates (SSUR) were plotted against OLRs and HRTs. Fig. 6 shows the SSUR values for COD at steady-state conditions HRTs between 5.40 and 480.3 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 Figs. 7 and 8. Maximum specific biomass growth rates (μmax) were in the range between 0.248 and 0.474 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 et al., 1980), 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.

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 9.

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

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5. 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 organic loading rates 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 minimize the effects on methanogens that might biogas production. Because methanogenesis is also 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, minimizes the build-up of inhibitory intermediates and stabilizes environmental conditions. This study adopted the mechanical mixing and biogas recirculation. Fig. 9 shows the gas production rate and the methane content of the biogas. The methane content generally declined with increasing OLRs. Methane gas contents ranged from 68.5% to 79% and the methane yield ranged from 0.29 to 0.59 CH4/g COD/d. Biogas production increased with increasing OLRs from 0.29 l/g COD/d at 0.5 kg COD/m3/d to 0.88 l/g COD/d at 9.5 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. 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 277.8 to 580 Litres per day during the study. In this scenario, the higher rate of carbon dioxide; (CO2) formation reduces the methane content of the biogas.

Figure 10.

Gas production and methane content

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6. Conclusions

The ultrasonic membrane anaerobic system, 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 were 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 by introduction of ultrasonic). The overall substrate removal efficiency was very high-about 98.5%. The gas production, as well as the 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 of anaerobic digestion at 30 oC 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 79%) that could be used to heat or produce hot water at the slaughterhouse wastewater plant.

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Nomenclature

COD: chemical oxygen demand (mg/l)

OLR: organic loading rate (kg/m3/d)

CUF: cross flow ultra-filtration membrane

SS: steady state

SUR: substrate utilization rate (kg/m3/d)

TSS: total suspended solid (mg/l)

MLSS: mixed liquid suspended solid (mg/l)

HRT: hydraulic retention time (day)

SRT: solids retention time (day)

SSUR: Specific substrate utilization rate (kg COD/kg VSS/d)

MAS: Membrane An aerobic System

UMAS: Ultrasonicated membrane anaerobic system

MLVSS: mixed liquid volatile suspended Solid (mg/l)

VSS: volatile suspended solids (mg/l)

MWCO: molecular weight Cut-Off

BLR: biological loading rate

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)

μmax: Maximum specific growth rate (day-1)

K: Maximum substrate utilisation rate (COD/g/VSS.day)

Ks: Half velocity coefficient (mg COD/l)

X: Micro-organism concentration (mg/l)

b=specific microorganism decay rate (day-1)

Y=growth yield coefficient (gm VSS/gm COD)

T: time

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Abdurahman Nour

Submitted: May 5th, 2014 Reviewed: August 26th, 2014