In recent years there has been a growing interest in anaerobic treatment of wastewaters. Compared to aerobic growth, anaerobic fermentation produces much less biomass from the same amount of COD removal. Alcohol distillery is highly water intensive units generating large volumes of high strength wastewater that poses serious environmental problems. Anaerobic digestion is the most suitable option for treatment of high strength organic effluent. The presence of biodegradable components in the effluents coupled with the advantages. Considerable progress has been achieved in the development of high rate anaerobic reactors with several configurations for treating concentrated industrial effluent. Considerable amount of studies have carried out using Hybrid up-flow anaerobic sludge blanket (HUASB) reactors. Treatment of spent wash generated from the distilleries is perceived as one of the serious pollution problem of the countries producing alcohol from the fermentation and subsequent distillation of sugar cane molasses. Distillery effluent is a contaminated stream with COD values of up to 80000-1, 30,000 mg/l and low pH Values of between 3 to 4. The HUASBR is widely used an effective step in removing the COD with a great efficiency.
This paper reviews the suitability and the status of development of anaerobic reactors for the digestion of selected organic effluents and critically analyzes the process parameter for reactors and main advantages of using HUASBR for treatment of distillery wastewater.
Keywords: HUASB reactor, anaerobic digestion, Distillery spent wash, Wastewater treatment parameters.
INTRODUCTION
One of the most important environmental problems faced by the world is management of waste. Industrial processes create a variety of wastewater pollutants; which are difficult and costly to treat. Wastewater characteristics and levels of pollutants vary significantly from industry to industry. Now-a-days emphasis is laid on waste minimization and revenue generation through byproduct recovery.
Rapid industrialization has resulted in the generation of a large quantity of effluent with high organic content, which if treated suitably, can result in a perpetual source of energy [2]. In recent years, anaerobic wastewater treatment has become a technology of growing importance, especially for highly polluted wastewater from the sugar & distillery industries [5]. Distillery spent wash refers to the effluent generated from alcohol distilleries. On an average 8-15 liters of effluent is generated for every liter of alcohol produced [1, 4]. India has around 319 distilleries; producing 3.25 billion liters of alcohol and generating 40.4 billion liters of wastewater annually [1]. The manufacturing process involves fermentation of diluted sugarcane molasses with yeast. The fermentation last about 80 hours and resulting product contains 6-8% alcohol. The yeast cells are separated by settling and cell free broth is steam distilled and rectified to obtain 94-95% alcohol [4]. The residue of fermented mash which comes out as liquid waste is termed as spent wash [1].
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The wastewater generated from distillation of fermented mash is in the temperature range 70-800c, deep brown in color, acidic in nature (low pH), and has high concentration of organic materials and solids. It is a very complex, caramelized and cumbersome agro industrial waste. However the pollution load of the distillery effluent depends on the quantity of molasses, unit operations for processing of molasses and process recovery of alcohols [1].
TABLE (1)
Typical characteristics of distillery spent wash [4].
Sr. No.
Parameter
Range
1.
pH
3.80-4.40
2.
Total Suspended Solids (mg/lit.)
12,000-14,000
3.
Total Dissolved Solids (mg/lit.)
58,000-76,000
4
Total volatile solids (mg/lit.)
45000-65000
5
B.O.D.,200C, 5 days (mg/lit.)
45,000-60,000
6
C.O.D. (mg/lit.)
80,000-1,30,000
7
Raw-colour
Dark- brown
8
Chlorides (mg/lit.)
5000-8000
Distillery spent wash has very high BOD, COD and high BOD/COD ratio. The amount of organic substances such as nitrogen, potassium, phosphates, calcium, sulphates is also very high.. High COD total nitrogen and total phosphate content of the influent may result in eutrofication of the natural water body. Disposal of the distillery spent wash on land is equally hazardous to the vegetation it is reported to reduce soil alkanity and manages availability, thus inhabiting seed germination. Application of distillery spent wash to soil without proper monitoring, seriously affects the ground water quality by altering its physiochemical properties such as color, pH, electric conductivity due to leaching down of organic and inorganic ions.
In spite of the fact of that there is the negative environmental impact associated with industrialization, the effect can be minimized and energy can be tapped by means of anaerobic digestion of the waste water [2]. Biological treatment of the distillery spent wash is eighter aerobic and anaerobic but in most cases the combination of both is used. A typical COD/BOD ratio of 1.8to1.9 indicates the suitability of influent of biological treatment [1].
In recent year considerable attention has been paid toward the development of reactor for anaerobic treatment of waste leading to conversion of organic molecule into biogas. This reactor known as second generation reactor or hi rate digester can handled waste at a high organic loading rate of 24kg. COD / m3 day and high up flow velocity of 2 mm/h at a low hydraulic retentions time [2].
Anaerobic digestion is the most suitable option for the treatment of high strength organic effluents. The presence of biodegradable components in the effluents coupled with the advantages of anaerobic process over other treatment methods makes it an attractive option.
1.1 Development of Anaerobic Reactors:
1. Septic Tank
2. Imhoff Tank
3. Single stage anaerobic reactors
4. Anaerobic Filter
5. Anaerobic Fluidized Bed Reactor
6. Upflow Anaerobic Sludge Blanket (UASBR).
HIGH RATE ANAEROBIC REACTORS
All modern high rate biomethanation processes are based on the concept of retaining high viable biomass by some mode of bacterial sludge immobilization. These are achieved by one of the following methods.
* Formation of highly settleable sludge aggregates combined with gas separation and sludge settling, e.g. upflow anaerobic sludge blanket reactor and anaerobic baffled reactor.
* Bacterial attachment to high density particulate carrier materials e.g. fluidized bed reactors and anaerobic expanded bed reactors.
* Entrapment of sludge aggregates between packing material supplied to the reactor, e.g. down flow anaerobic filter and up flow anaerobic filter.
2.1. Fixed film reactor:
In stationary fixed film reactors (Fig. 1), the reactor has a bio-film support structure (media) such as activated carbon, PVC (polyvinyl chloride) supports, hard rock particles or ceramic rings for biomass immobilization. The wastewater is distributed from above/below the media. Fixed film reactors offer the advantages of simplicity of construction, elimination of mechanical mixing, better stability at higher loading rates, and capability to withstand large toxic shock loads and organic shock loads. The reactors can recover very quickly after a period of starvation. The main limitation of this design is that the reactor volume is relatively high compared to other high rate processes due to the volume occupied by the media. Another constraint is clogging of the reactor due to increase in bio-film thickness and/or high suspended solids concentration in the wastewater [2].
Feed storage
tank
Feed
TABLE (2)
Characteristics of reactor types [4].
Anaerobic Reactor Type
Start up period
Channeling Effect
Effluent Recycle
Gas solid separation Device
Carrier Packing
Typical Loading rates (kg COD/m3day)
HRT (d)
CSRT
—
Not Present
Not required
Not required
Not essential
0.25-3
10-60
UASB
4-16
Low
Not required
Essential
Not essential
10-30
0.5-7
Anaerobic Filter
3-4
High
Not required
Beneficial
Essential
1-4
0.5-12
AAFEB
3-4
Less
Required
Not required
Essential
1-50
0.2-5
AFB
3-4
Non-existent
Required
Beneficial
Essential
1-100
0.2-5
2.2. Up flow anaerobic sludge blanket reactor:
UASB technology is being used extensively for effluents from different sources such as distilleries, food processing units, tanneries and municipal wastewater. The active biomass in the form of sludge granules is retained in the reactor by direct settling for achieving high MCRT thereby achieving highly cost-effective designs. A major advantage is that the technology has comparatively less investment requirements when compared to an anaerobic filter or a fluidized bed system.
Among notable disadvantages, it has a long start-up period along with the requirement for a sufficient amount of granular seed sludge for faster startup. Moreover, significant wash-out of sludge during the initial phase of the process is likely and the reactor needs skilled operation.
A UASB reactor (fig. 2) essentially consists of gas-solids separator (to retain the anaerobic sludge within the reactor), an influent distribution system and effluent draw off facilities. Effluent recycle (to fluidize the sludge bed) is not necessary as sufficient contact between wastewater and sludge is guaranteed even at low organic loads with the influent distribution system. Also, significantly higher loading rates can be accommodated in granular sludge UASB reactors as compared to flocculent sludge bed reactors. In the latter, the presence of poorly degraded or no biodegradable suspended matter in the wastewater results in an irreversible sharp drop in the specific methanogenic activity because the dispersed solids are trapped in the sludge. Moreover, any significant granulation does not occur under these conditions. The maximum loading potential of such a flocculent sludge bed system is in the range of 1-4 kg COD/m3 day. Yet another high rate digester, EGSB, is a modified form of UASB in which a 5-10 m/h as compared to 3 m/ h for soluble wastewater and 1-1.25 m/h for partially soluble slightly higher superficial liquid velocity is applied wastewater in an UASB). Because of the higher up flow velocities, mainly granular sludge will be retained in an EGSB system, whereas a significant part of granular sludge bed will be in an expanded or possibly even in a fluidized state in the higher regions of the bed. As a result, the contact between the wastewater and sludge is excellent. Moreover, the transport of substrate into the sludge aggregates is much better as compared to situations where the mixing intensity is much lower. The maximum achievable loading rate in EGSB is slightly higher than that of an UASB system, especially for a low strength V&A containing wastewater and at lower ambient temperatures.
Fig.2 UASB Reactor.
2.3. Anaerobic fluidized bed reactor:
In the anaerobic fluidized bed (Fig. 3), the media for bacterial attachment and growth is kept in the fluidized state by drag forces exerted by the up flowing wastewater. The media used are small particle size sand, activated carbon, etc. Under fluidized state, each media provides a large surface area for biofilm formation and growth. It enables the attainment of high reactor biomass hold-up and promotes system efficiency and stability. This provides an opportunity for higher organic loading rates and greater resistance to inhibitors. Fluidized bed technology is more effective than anaerobic filter technology as it favors the transport of microbial cells from the bulk to the surface and thus enhances the contact between the microorganisms and the substrate.
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Fig. 3 Anaerobic fluidized bed reactor
These reactors have several advantages over anaerobic filters such as elimination of bed clogging, a low hydraulic head loss combined with better hydraulic circulation and a greater surface area per unit of reactor volume. Finally, the capital cost is lower due to reduced reactor volumes. However, the recycling of effluent may be necessary to achieve bed expansion as in the case of expanded bed reactor. In the expanded bed design, microorganisms are attached to an inert support medium such as sand, gravel or plastics as in fluidized bed reactor. However, the diameter of the particles is slightly bigger as compared to that used in fluidized beds. The principle used for the expansion is also similar to that for the fluidized bed, i.e. by a high up flow velocity and recycling.
2.4 The Anaerobic filter Processes (AF):
Biofiltration uses bacterial immobilization by means of slime of films on an inert support material & the entrapment of sludge flocs within the macro-porous structure of the carrier material to retain as much of the active sludge as possible.
Especially designed carrier materials are available, usually made of polyethylene or polypropylene. They are highly voided to reduce the risk of clogging & have specific surface between 100&200 m2per m3carrier materials.
Anaerobic filter are used whenever non-granular or non settable sludge is expected & when available area is limited. The high biomass concentration inside the reactor allows volumetric loading rates of 5to10kg COD/m3per day. A disadvantage of the Anaerobic Filter is the relative high cost of the carrier material.
2.5 The Hybrid Reactor:-
Hybrid Type of reactor is a combination of an Up flow Anaerobic Sludge Blanket reactor with an anaerobic filter or an anaerobic contact process or a combination of the three types.
The first hybrid Type of reactor is similar to an UASB, except for the three-phase separator. The separator is replacing by a later of floating carrier material. This material serves a double function
(1) To separate & retain a large functions of sludge in the reactor before the influent use the reactor, and
(2) To carries active sludge in the porous space of the carrier material itself. This type of reactor is called the up flow anaerobic contact filter reactor (UACF)
The second type of hybrid reactor has recently been developed for waste water showing no granule formation & requiring a longer hydraulic retention time. It is called by up flow Anaerobic contact reactor (UAC).This reactor allows some bio mass accumulation in the lower part of the reactor the reactor is not totally mix which is case for the anaerobic contact (AC) reactor but is equipped with a sophisticated influent distribution system similar to the one for the (UASB) reactor [5].
3. CONTROL OF ANAEROBIC DIGESTION
The anaerobic digestion process is affected significantly by the operating conditions. As the process involves the formation of volatile acids, it is important that the rate of reaction be such that there is no accumulation of acids, which would result in the failure of the digester. This, in turn, is governed by the loading rate and the influent strength. Temperature and pH are other important variables as the methane producing bacteria are sensitive to these as well.
3.1. Effect of temperature
Anaerobic digestion is strongly influenced by temperature and can be grouped under one of the following categories: psychrophilic (0-20°C), mesophilic (20- 42°C) and thermophilic (42-75°C). The details of the bacterial processes in all the three temperature ranges are well established though a large section of the reported work deals with mesophilic operation. Changes in temperature are well resisted by anaerobic bacteria, as long as they do not exceed the upper limit as defined by the temperature at which the decay rate begins to exceed the growth rate. In the mesophilic range, the bacterial activity and growth decreases by one half for each 10°C drop below 35°C.Thus, for a given degree of digestion to be attained, the lower the temperature, the longer is the digestion time. The effect of temperature on the first stage of the digestion process (hydrolysis and acidogenesis) is not very significant. The second and third stages of decomposition can only be performed by certain specialized microorganisms (acidognic and methanogenic bacteria) and thus, these are much more sensitive towards temperature change [3]. However, an important characteristic of anaerobic bacteria is that their decay rate is very low at temperatures below 15°C. Thus, it is possible to preserve the anaerobic sludge for long periods without losing much of its activity. This is especially useful in the anaerobic treatment of wastewater from seasonal industries such as sugar mills.
3.2. Effect of pH
Anaerobic reactions are highly pH dependent. The optimal pH range for methane producing bacteria is 6.8-7.2 while for acid-forming bacteria, a more acid pH is desirable. The pH of an anaerobic system is typically maintained between methanogenic limits to prevent the predominance of the acid-forming bacteria, which may cause V&A accumulation. It is essential that the reactor contents provide enough buffer capacity to neutralize any eventual V&A accumulation, and thus prevent build-up of localized acid zones in the digester. In general, sodium-bicarbonate is used for supplementing the alkalinity since it is the only chemical, which gently shifts the equilibrium to the desired value without disturbing the physical and chemical balance of the fragile microbial population.
3.3. Effect of nutrients
The presence of ions in the feed is a critical parameter since it affects the granulation process and stability of reactors like USAB. The bacteria in the anaerobic digestion process requires micronutrients and trace elements such as nitrogen, phosphorous, sulphur, potassium, calcium, magnesium, iron, nickel, cobalt, zinc, manganese and copper for optimum growth. Although these elements are needed in extremely low concentrations, the lack of these nutrients has an adverse effect upon the microbial growth and performance. Methane forming bacteria have relatively high internal concentrations of iron, nickel and cobalt. These elements may not be present in sufficient concentrations in wastewater streams from the processing of one single agro industrial product like corn or potatoes or the wastewater derived from condensates. In such cases, the wastewater has to be supplemented with the trace elements prior to treatment. The required optimum
C: N: P ratio for enhanced yield of methane has been reported to be 100:2.5:0.5. The minimum concentration of macro and micronutrients can be calculated based on the biodegradable COD concentration of the wastewater, cell yield and nutrient concentration in bacterial cells. The nutrient
Concentration in the influent should be adjusted to a value equal to twice the minimal nutrient concentration required in order to ensure that there is a small excess in the nutrients needed.
3.4. Effect of organic loading rate
In anaerobic wastewater treatment, loading rate plays an important role. In the case of nonattached biomass reactors, where the hydraulic retention time is long, overloading results in biomass washout. This, in turn, leads to process failure. Fixed film, expanded and fluidized bed reactors can withstand higher organic loading rate. Even if there is a shock load resulting in failure, the system is rapidly restored to normal. In comparison to a CSTR system, fixed film and other attached biomass reactors have better stability. Moreover, high degree of COD reduction is achieved even at high loading rates at a short hydraulic retention time. Anaerobic fluidized bed appears to withstand maximum loading rate compared to other high rate reactors.
4. FACTORS GOVERNING REACTOR CHOICE
A technology is acceptable to an industry if it requires less capital, less land area and is more reliable when compared to the other well established options for an anaerobic digestion system; this translates into the process being able to run at high organic and hydraulic loading rates with minimum operation and maintenance requirements. To choose the most appropriate reactor type for a particular application, it is essential to conduct a systematic evaluation of different reactor configurations with the wastewater stream. The organic and hydraulic loading potential of a reactor depends on three factors
Viz: *€ Amount of active biomass that can be retained by a reactor per unit volume.
* Contact opportunity between the retained biomass and the incoming wastewater.
* Diffusion of substrate within the biomass.
With these considerations, granular sludge UASB reactor stands out distinctively as the best choice with the only limitations being the tendency of granules to float and shearing of granules at high loading rates. These constraints are also valid to a lesser degree for attached biomass reactors (such as fixed film, fluidized bed and rotary biological contactors). In addition, due to the space occupied by the media, the attached biomass reactors possess comparatively lower capacity for biomass retention per unit volume of the reactor. The latter depends on the film thickness, which would be the highest in a fluidized bed reactor due to large surface area available for biomass attachment. Also, there is better contact between the biomass and the incoming wastewater in both fluidized bed and EGSB systems. However, due to the high upflow velocity, the substrate diffusion in the biomass is limited in these configurations.
Based on these factors, it appears that the maximum achievable loading rates with soluble wastewater would decrease in the following sequence:
UASB > EGSB > fluidized bed reactor > anaerobic filter. The capital cost of the reactors and the land area requirements, therefore, follows the same order. The digester operation and maintenance requirements are minimum if the process is fairly stable towards fluctuations in wastewater characteristics and changes in environmental conditions. Susceptibility of the process depends on the potential utilization of the reactor and thus a system operating near maximum loading conditions is more sensitive. Based on the comparisons of various reactor types, the following order can be recommended for reactor choice:
Parameters Rating
Operating skills: Fixed film < UASB < RBC <
Fluidized bed.
Energy consumption: UASB < fixed film < EGSB < fluidized bed < RBC
Capital cost, land requirement: RBC < fixed film < UASB < EGSB < fluidized bed
5. HUASB REACTOR & ITS PERFORMANCE:
The hybrid up flow anaerobic sludge blanket (HUASB) reactor has received widespread acceptance and has been successfully used to treat a variety of industrial as well as domestic wastewaters. In the HUASB process, the whole waste is passed through the anaerobic reactor in an up flow mode, with a hydraulic retention time (HRT) of only about 8-10 hours at average flow. No prior sedimentation is required.COD removal efficiencies depends largely on wastewater type; however the removal efficiency with respect to biodegradable COD is generally in excess of 85 or even 90%.
The biodegradable COD is sometimes reflected in the parameter biological oxygen demand (BOD). The four top applications of high rate anaerobic reactor systems are for:
Breweries & beverage industry.
Distilleries and fermentation industries.
Food industries.
Pulp & paper industries.
Furthermore in warm climate the HUASB concept is also suitable for the domestic wastewater.
Advantages of Anaerobic Reactors:
Low energy cost
Less bio-mass generation
Less solid waste to dispose
Stable digested sludge is produced
Less space required
Off-gas air pollution eliminated
Limitations of HUASBR:
*Post Aerobic Treatment is required (one day polishing pond for sewage).
*To meet coli form level in the treated effluent maturation pond or chemical treatment is required.
CONCLUSION
A brief summary of results of laboratory and pilot scale studies extracted from expensive literature survey are presented. The HUASBR technology is well suited for the pre-treatment of high strength distillery effluents. It must be noted that this is only when the process has been successfully started up and it is in stable operation. It order to achieve a successive start up it is recommended that the reactor be started up at a low loading rate between 4-8 Kg.COD/m3.day and the COD removal efficiency must be monitored carefully. Attention must also be paid to the temperature and high loading rate should not be applied until the temperature in the reactor has reached the recommended 34 to 360c.This especially important in effluent steams that have low flow rate with correspondingly high COD concentration such as distillery waste. Once the plant has been successfully started up, fluctuations in volumetric loading rate do not significantly affect the performance of the reactor.
ACKNOWLEDGMENT
The literature reviewed in this paper is the part of ongoing thesis work name “Study on performance of Tapered conical shaped hybrid Upflow anaerobic sludge blanket Reactor (HUASBR) for treatment of distillery spent wash” at SGB University, Amravati under the guidance of Dr. N. W. Ingole. The author thanks the Principal, J. T. M. C. O. E. Faizpur, Dist- Jalgaon for extending all facilities for conducting the research work.
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