Performance Of Uasb Reactors In Treating Sewage Wastewater Biology Essay

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Disposal of sewage has become more problematic over recent years because of stricter environmental regulation that controls the disposal of raw and untreated sewage. The awareness of environmental problems has forced governments, local authorities and utilities for management to search for new alternative process or solutions for future wastewater management strategies. To increase the efficiency of biological wastewater treatment processes, a variety of engineering solutions have been developed in recent years. Treatment process reliability is of particular importance for wastewater reuse applications. While high rate processes are efficient and cost-effective for treatment of wastewater, their reliability is highly dependent on specific wastewater characteristics. The principal objective of biological wastewater treatment is to remove organic contaminants. However, the contaminants that must be removed from wastewater are complex mixtures of particulate and soluble inorganic and organic constituents.

Anaerobic digestion of sewage wastewater using microorganisms is an effective means of generating methane rich biogas-a potential source of energy. Anaerobic digestion technology for methane production from biomass has been known since the 17th century. However, the failure rate for anaerobic digesters is still very high (Lusk, 1998). These failures are believed to be mainly due to poor design, construction, operation, as well as inadequate mixing. Digester mixing is needed to break scum, to homogenize the contents of digesters (substrate and microorganisms, temperature, and pH).

High-rate anaerobic treatment is an attractive process for domestic sewage because of its low construction, operation and maintenance costs, small land requirement, low excess sludge production and production of biogas. Although the anaerobic treatment of domestic sewage has been applied at large scale in several tropical countries, the process so far is not applied at full scale in countries with lower temperatures. This is mainly due to lower removal efficiencies. Also, at low temperatures, a longer hydraulic retention time (HRT) is needed due to the lower rate of hydrolysis and the higher amount of accumulating suspended solids (SS) in the reactor (Zeeman and Lettinga, 1999). Up-flow anaerobic sludge blanket (UASB) reactors inoculated with granular sludge give a better performance than reactors inoculated with flocculent sludge (Lettinga et al., 1983). However, when treating domestic sewage in granular sludge UASB reactors at low temperatures, accumulation of SS occurs in the granular sludge bed, which leads to deterioration of the overall methanogenic activity and the reactor performance (Uemura and Harada, 2000). Several investigators (Elmitwalli et al., 2000; Kalogo and Verstraete, 1999; Last and Lettinga, 1992) revealed that at low temperatures pre-removal of SS is needed prior to anaerobic treatment in granular sludge bed reactors.

Anaerobic digestion is a complex biological process regulated by a bacteria consortium, which is sensitive to the type of substrate and its composition. Specific substrates are utilized by specific types of bacteria, whilst exerting a symbiotic influence on one another's activity forming an anaerobic food chain (Gavala and Lyberatos, 2001; Angelidaki and Sanders, 2004; Dearman et al., 2006). Substrates are hydrolyzed to sugars, amino acids, and fatty acids which are further consumed by acetogens excreting acetate, hydrogen, and carbon dioxide. These simplistic compounds are consumed readily by methanogens, co-existing with the acetogens. Thus, by anaerobic treatment of wastewaters is achieved not only removal of pollutants of the wastewater streams but also the production of renewable energy in the form of methane.

Up-flow anaerobic sludge blanket (UASB) reactor is desirable in high-strength organic wastewater treatment because of its high biomass concentration and rich microbial diversity. The high biomass concentration implies that contaminant transformation is rapid, and highly concentrated or large volumes of organic waste can be treated in bio-reactors. the UASB system is highly dependent on its granulation process with a particular organic wastewater. Anaerobic granular sludge is the core component of an UASB reactor. Sludge granules are dense, multi-species, microbial communities, and none of the individual species in the granular ecosystem is capable of degrading complex organic wastes. One major drawback of the UASB reactor is its extremely long start-up period, which generally requires 30 to 60days for the development of anaerobic granular sludge.

The main aim of this present work was to study the effect of different organic loading rate (OLR) and (Hydraulic retention time) HRT on chemical oxygen demand (COD) reduction in a UASB reactor treating sewage wastewater under mesophilic temperature range.

Material and methods

Up flow anaerobic sludge blanket (UASB) reactor were used to treat the sewage wastewater and the study was carried out in order to find out the efficiency of UASB for the treatment of sewage wastewater on lab scale. Sewage wastewater samples were collected twice a week from inlet of septic tank of boys hostel and are kept at 4 0C in refrigerator for further study. The bacterial culture inoculums used in the experiment were obtained from Haryana Associate Distillery, and sewage sludge from septic tank. The physico-Chemical analysis of all parameters was carried out as per standard methods.

Cylindrical lab- scale UASB reactors were constructed made of Acryilic plastic and the effective volume of the reactor was 15L having a height of 60 cm and diameter of 19.2 cm. The reactor was provided with an inlet distribution system at the bottom along with five sampling ports for sample collection. The treated sewage wastewaters were collected from the 5th port. U tube was connected to the sampling port 5th in order to prevent any leakage of biogas from reactors. A peristaltic pump (Master Flex L/S Model) with different flow rates was used to pump the sewage raw wastewater into the reactor. Reactor was placed at room temperature ranging from 20 - 40 0C. All the connections were made leak proof by using silicon tubing. The systematic diagrams are shown in fig. 1

Fig. 1 Systematic line diagram of experiment

The start up period is considered as the period for stable operation to be achieved before starting the reactor in continuous mode. Initially all the reactors were run under batch mode for 90 days in order to acclimatize the sludge and after complete acclimatization reactors were run at continuous mode to asses the effect of OLR and HRT on COD reduction. Before starting the UASB reactors in continuous mode, the bacterial inoculums present in UASB & sewage sludge was acclimatized for this purpose. The UASB reactors were seeded with mixture 4.5 L of UASB sludge and sewage sludge & rest volume of reactors were filled with sewage wastewater. The total volume of liquid in the reactor was 15 L. The reactors were kept at room temperature for two weeks. After two weeks 500ml sample of effluent was collected from the sampling port no.4 for analysis of different parameters. The same volume of sewage influent was fed to the reactors on weekly basis for 90 days and after 90 days about 80% reduction in COD was observed and the reactor was run in continuous mode.

This study was carried at room temperature ranging from 20 - 40 0C. UASB reactors were fed with sewage waste water at HRT of 24 hours, 48 hours and 72 hours. In first phase of the study the reactors was fed with sewage wastewater, the flow rate was maintained 10ml/min (OLR= 0.13 Kg/d/m3) with the help of peristaltic pump. The reactors were run for 10 days at HRT of 24 hours. In the second phase, the flow rate was maintained 5 ml/min (OLR= 0.068 Kg/d/m3) at 48 hours HRT for 10 days. In third, phase the reactors were maintained at the flow rate of 3ml/min (0.045 Kg/d/m3) for furtheR10 days at 72 hrs HRT as given in table 2.

Calculations

Specific methanogenic activity

Specific methanogenic activity (SMA) test or Specific sludge activity, which evaluates the anaerobic sludge capability to convert an organic substrate into methane, which escapes easily to the gas phase carrying with it reducing equivalents that cause chemical oxygen demand (COD) or biochemical oxygen demand (BOD) in the liquid phase.

Besides being a parameter for evaluation of the efficiency of the treatment, the SMA is also used to evaluate the sludge activity during different operational steps of an anaerobic system. The seed sludge activity is an important parameter to asses the evolution of the anaerobic reactor start-up, (26). SMA was calculated with the help of equation (i) given by (3).

(i)

Where,

Q = feeding flow rate (L/d)

B = Biomass in the reactor (gVSS)

Quantification Methane

The methane produced during the period of study was quantified by the volumetric method with biogas wash in NaoH (15%). The NaoH (15%) was used. Sodium hydroxide solution was filled in the displacement bottles for methane measurement all bottles were sealed and air tight. Bottles were connected with reactors. As biogas produced inside the reactors, it exerts a pressure on the NaoH (15%) and displaced the solution by equal volume of gas. Carbon dioxide dissolve in the alkaline liquid forming the carbonates, bicarbonates and biogas was stored at the top of the bottle by displacing the sodium hydroxide from the bottle. The amount of liquid displaced from the bottle is equivalent to the amount of biogas produced.

Quantification of carbon dioxide

The displaced NaOH (15%) were collected in the measuring cylinder, it contains the carbon dioxide in the forms of carbonate and bicarbonates salts. The estimation of carbonates and bicarbonates were calculated with the help of below given equation (ii),

(ii)

Sludge Retention Time

It is the average time in days for which biomass or biological solids are retain or remains in the biological reactor or system. It is also called sludge age or mean cell residence time. It is measured as biomass or solids of the reactor or the system per unit of mass of solids removed or washed from the reactor or the system every day. For a system without return of sludge to the reactor is equal to the HRT. SRT is calculated with the following formula:-

Sludge Retention Time = (iii)

Where,

V = Volume, mL

X = Concentration of microorganism in reactors

Qe = Disharge, m3/sec

Xe = Concentration of microorganism in effluent

Qw = wasted Discharge

sXw = Sludge wastage

Maximum removal of COD

The removal efficiency of COD is calculated on the basis of following formula:-

(iv)

Where,

CODinf = Influent COD

CODeffl = Effluent COD

CODT = Influent total COD

Chemical analysis

Chemical Oxygen Demand (COD), Volatile Fatty Acids (VFA), Alkalinity and pH of the raw and treated wastewater were analysed by using (2). Biogas production was also measured during course of treatment.

Result and discussion

The UASB reactors were operated at a different OLR of 0.13, 0.068 and 0.045 Kg COD/m3/d with HRT varying from 24hrs, 48hrs and 72hrs respectively. The overall removal efficiency of two reactors was 81% for CODtotal for R1 and R2 respectively. This relatively good performance could be attributed to the low OLR and different HRT's, which would effectively increase the efficiency of hydrolysis and subsequent digestion of organic matter. COD was measured on oxygen equivalent of the organic matter present in wastewater. COD of reactors R1 and R2 varies from 662 to 80 mg/L at HRT from 24hrs to 72hrs respectively.

The percentage removal of COD in reactors R1 and R2 was 75%, 81% and 87% at HRT 24hrs, HRT 48hrs and HRT 72hrs respectively. The overall reduction in COD of reactors R1 and R2 was 81% respectively. This may be due to enzymatic hydrolysis promoted a slight improvement on the COD removal. Anaerobic reactions are highly pH dependent. The optimum pH range for methane producing bacteria is 6.8 to 7.2. Higher pH is desirable for acid forming bacteria, the pH of anaerobic UASB reactor is typically maintained between methanogenic limit to prevent the predominance of acid formers bacteria, which may cause VFA accumulation (7). So, it is essential that the reactor contents provide enough buffer capacity to neutralize VFA accumulation. The effect of pH change in the inefficient can be buffered by the alkalinity i.e available in the reactor in case of sudden drastic pH changes; the efficiency of the reactor may increase or may decrease. The average pH value of raw sewage was 7.4. The average pH of reactors R1 & R2 in phase I, II & III are 7.11 & 7.05, 7.12 & 7.12 & 6.76 and 6.77 respectively (as shown in figure 3).

The biogas produced during the anaerobic degradation is a valuable resource of energy. The quality of the biogas has special importance. Biogas contains about 65-70% methane by volume with remaining 30-35% being almost totally carbon dioxide. The cumulative production of CH4 in displacement bottles is given in figure 4.

VFA of reactor R1 varies from 9028 to 6502 mg/L and 9155 to 3960 mg/L for reactor R2 respectively from start of experiment to the end of experiment respectively (as shown in figure 5). The VFA concentration was decreased during treatment process. Since the VFA released through the degradation of complex or simple organic solids converted to acetate and hydrogen which may ultimately converted to the methane through this process. The low pH value observed in this study could be attributed to the production of low alkalinity which is not enough for maintaining the natural pH and buffering the VFA produced. The sudden rise in the reactors might be due to the rapid utilization of VFA by phosphate accumulating organisms (PAOs) for synthesis of their stored products (23). The high rate of VFA utilization may due to higher activity of micro-organisms.

Alkalinity of reactor R1 varies from 900 to 1900 mg/L, 1250 to 1450 mg/L and 1050 to 1850 mg/L at HRT 24hrs , 48hrs and 72hrs respectively. In R2 alkalinity varies from 900 to 1850 mg/L, 850 to 1650 and 1150 to 2650 mg/L at HRT 24hrs , 48hrs and 72hrs respectively (as shown in figure 6).

VSS of reactor R1 varies from 15 to 70 mg/L, 10 to 30 mg/L and 10 to 18 mg/L for HRT 24hrs, 48hrs and 72hrs respectively. VSS of reactor R2 varies from 10 to 70 mg/L, 12 to 32 mg/L and 8 to 25 mg/L for HRT 24hrs, 48hrs and 72 hrs respectively (as shown in figure 7).

The maximum removal efficiency of VSS for R1 was 86, 91.2, 91% when HRT was 24, 48 and 72 hours respectively and in R2 it were 91.2, 89 and 93% respectively. The decrease in concentration of VSS was observed when the process moved from acidogenic to methanogenic stage. The rate of removal of VSS varies with the HRT. The VSS mainly removed from the reactor was converted to methane and rest settles down into the sludge. The average concentrations of influent and effluent of different parameters are shown in table 3 given below.

Specific methanogenic Activity

The specific sludge activity of reactor R1 varies from 0.16, 0.18 and 0.19 g/d at HRT of 24hrs, 48 hrs and 72 hrs respectively. Similarly the specific sludge activity of reactor R2 varies from 0.14, 0.15 and 0.15 g/d at HRT of 24 hrs, 48 hrs and 72 hrs respectively.

The result obtained in this investigation revealed that the UASB reactors is an efficient technique for treatment of sewage wastewater at thermophillic conditions but is subjected to daily fluctuations in influent concentrations. The main objectives of the present study were anaerobic treatment of sewage wastewater by using Up-flow Anaerobic Sludge Blanket (UASB) reactors under thermophilic conditions and assess the methanogenic activity from the waste. The Specific Methanogenic Activity from sewage wastewater also studied as gCOD/gVSS.d. The overall increase in reduction of the various parameters like COD, VSS, and VFA occurs; while pH and alkalinity remained unchanged or shows slight increase with increasing HRT.

Conclusions

The result obtained in the work show that with increase in HRT increases the biological conversion in UASB reactors applied to sewage wastewater. Thus, we can conclude that the anaerobic treatment of sewage wastewater by UASB technology is a feasible option for discharge of treated wastewater into receiving water body. It is recommended that the use of aerobic treatment units after UASB reactors increases the overall removal efficiency meeting the standards to discharge the treated wastewater into the receiving water body.

Table 1: Characteristics of Domestic Wastewater under study

Parameters Values

pH

COD

Total Suspended Solids (TSS)

Volatile suspended solids (VSS)

Total Dissolved Solids (TDS)

Volatile Fatty Acids (VFA)

TKN

Alkalinity

Sulphate

Phosphate

7.4

662

145

114

1468

3825

34

794

91.85

37.59

Note: All values are in parts per million except pH & ratios

Table 2: Experimental protocol

Reactors

Hydraulic Retention Time ( HRT)(Hours)

Up-flow velocity

(m/sec)

Flow

(m3/sec)

Organic Loading Rate

(OLR) (KgCOD/d)

R1

24

48

72

0.020

0.010

0.006

10

5

3

0.13 Kg/d/m3

0.068 Kg/d/m3

0.045 Kg/d/m3

R2

24

48

72

0.020

0.010

0.006

10

5

3

0.13 Kg/d/m3

0.068 Kg/d/m3

0.045 Kg/d/m3

Table 3: Physco-chemical characteristics of UASB reactors

Period

Phase I

Phase II

Phase III

R1

R2

R1

R2

R1

R2

Influent

pH

CODT

TSS

TDS

VSS

VFA

TKN

ALK

So42-

Po43-

7.4

662

145

1468

114

3825

34

794

92

38

7.4

662

145

1468

114

3825

34

794

92

38

7.4

662

145

1468

114

3825

34

794

92

38

7.4

662

145

1468

114

3825

34

794

92

38

7.4

662

145

1468

114

3825

34

794

92

38

7.4

662

145

1468

114

3825

34

794

92

38

Effluent

pH

CODT

TSS

TDS

VSS

VFA

TKN

ALK

So42-

Po43-

7.4

188

98

1833

29

7548

10

1145

91

36

7.4

220

84

2397

28±21

6463

10

1190

91

36

7.2

132

30

1745

24

7108

5

1280

83

30

7.3

128

31

1973

24

3466

5

1330

8

29

7.4

116

57

1097

13

3320

2

1409

59

17

7.3

124

56

1192

16

2077

2

1675

58

16

Conversion

%CODT

%VSS

CH4production

75%

86%

9.96%

75%

91%

6.76%

81%

91.2%

24.98%

81%

89%

19.5%

87%

91%

65.05%

87%

93%

73.7%

Figure 1: COD value of reactors R1 & R2 at different HRT

Figure 2: Total Alkalinity of reactors R1 & R2

Figure 3: VSS concentration of reactors R1 & R2

Figure 4: VFA concentration of reactors R1 & R2

Figure 5: Cumulative methane production (ml) of reactors R1 & R2

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