For treating dairy effluents, there are aerobic and anaerobic processes, such as oxidation ponds, aerobic lagoons, activated sludge process, bio filters, root zone absorption, anaerobic filters and sludge blanketing process. The Up-flow Anaerobic Sludge Blanket process (UASB) has been found to be one of the most successful ways for primary treatment of dairy effluent where biological oxygen demand (BOD) load can be reduced to 90 per cent.

In the UASB process, dairy effluent is fed from the bottom of a reactor and flows upward through the biologically formed sludge blanket comprising tiny particles. Various gases produced under anaerobic conditions cause internal circulation, which helps in formation and maintenance of biological sludge. Some of the gas produced within the sludge blanket gets attached to the biological granules. The free gas and the particles with the attached gas rise to the top of the reactor and gets collected for further use.

Biogas produced in a UASB reactor contains by volume 65-80 per cent methane, 15 to 25 per cent carbon dioxide and traces of nitrogen, hydrogen, sulphur dioxide, moisture etc. Advantages of UASB process are:

  • Biogas can be used for heating and generation of electricity
  • Organic matter is almost fully degraded and reduces sludge
  • Emission of greenhouse gases is reduced.
  • A clean environment is obtained as the obnoxious odours are almost eliminated.
  • Clean treated water produced for irrigation or other uses.
  • The system has ability to take shock loads


Essential components of a UASB system

Oil/grease is removed from raw effluent, after which the effluent is sent to a collection tank from where it goes to an equalisation tank. A primary clarifier is employed to remove the floating scum and the left over oil. The effluent is then led to the buffer tank to feed the UASB reactor. Part of the treated effluent from the outlet of reactor is re-circulated to maintain the suspended sludge in the reactor.

Usually, anaerobic treatment is followed by aerobic treatment, secondary treatment and finally clarified to get the treated effluent of desired parameters (Figure 1). Sludge settled at the bottom of the secondary clarifier is re-circulated to aeration tank so as to maintain micro-flora. The excess sludge, if any, is taken to sludge drying beds.

Figure 1: UASB treatment scheme for dairy effluent


Anaerobic digester

Anaerobic digester (Figure 2) is the heart of UASB system. The organic material in the effluent is decomposed in an anaerobic condition by micro-flora that lives in oxygen free environment. During this process, organic nitrogen compounds are converted to ammonia, sulfur compounds to hydrogen sulfide, phosphorus to orthophosphates; and calcium, magnesium, and sodium to various salts. End products of anaerobic digestion are: natural gas (mainly methane), a nutrient rich organic sludge and inorganic products. There are three main groups of bacteria involved in anaerobic digestion. These are: hydrolyzing bacteria (solubilizing bacteria), acetogenic bacteria (acetate forming bacteria), and, methanogenic bacteria (methane forming bacteria). The digestion process is divided into four sub processes:


Figure 2: Anaerobic digester/reactor


  1. Hydrolysis process: The insoluble constituents such as carbohydrate, fat, protein etc. are converted into soluble organic constituents such as simple sugars, fatty acids, and amino acids. This is carried out by hydrolyzing bacteria through the release of extra-cellular enzymes.
  2. Acidogenesis process: At this step the products obtained are converted to lower molecular intermediate products such as alcohol, butyric acid, propionic acid, ammonia, hydrogen, carbon dioxide, hydrogen sulfide and a variety of short chain volatile organic acids.
  3. Acetogenesis process: At this step, acetogenic bacteria convert the fermentation products in earlier steps into acetic acid, acetate, hydrogen and carbon dioxide.
  4. Methanogenesis process: This is the terminal stage of anaerobic digestion, wherein methanogenic bacteria are active. Methanogens are divided into two major groups: the acetate converting (acetotrophic) bacteria and the hydrogen utilizing (hydrogenotrophic) bacteria. Acetotrophs convert volatile acids and intermediate products to final product viz., methane, carbon dioxide and water. Hydrogenotrophic group reduces the hydrogen toxicity by scavenging hydrogen to produce ammonia, hydrogen sulfide and methane.

Chemical oxygen demand (COD) removal does not take place during hydrolysis / acidogenic / acetogenesis process. The actual elimination of organic matter occurs in the methanogenic step in which the COD in the form of methane is separated from the effluent.


Critical factors affecting functioning of UASB system

The two major factors that hinder proper performance of UASB system are prevention of floating fat from entering the reactor and maintenance of Volatile Suspended Solids (VSS) level in the reactor. Entry of fat into the reactor causes destruction of micro flora leading to its malfunctioning. It is therefore necessary that oil and grease are trapped before collection tank. Occurrence of VSS in the reactor is the direct measure of bacterial mass in reactor. It should be maintained for optimum performance.

Most important parameters for optimum performance of the reactor are discussed below: 

  1. pH: It is important to maintain the pH in the neutral range of 6.5 to 7.5 for optimum methanogeic activity. In case it drops below 6.5, acidifying bacteria will continue to acidify and methane formation would stop. The pH needs to be adjusted before the effluent enters the reactor. However, pH upto 8.0 for short period was found not affecting the performance significantly.
  2. Temperature: Methanogenic bacteria perform best in mesophilic range. It is recommended to maintain the temperature at 35o-37oC for optimum performance. The temperature inside the reactor is also determined by the temperature of the effluent and the atmosphere.
  3. Nutrient: Anaerobic bacteria need nutrients for their growth. Lack of nutrients will reduce the formation of sludge and removal of COD. At the start up urea and diammonium phosphate may be added. There is no need to add supplements after the reaction starts because biological sludge generated from dairies is a rich source of nutrients for bacteria. The need for micro nutrients can be met by adding cow dung in the buffer tank.


Energy savings

If the biogas produced is utilised for power generation, the UASB system helps reduce the power use by 40 per cent. The process is environment friendly. Energy consumption for BOD/COD reduction is absolutely nil. Biogas from UASB reactor accumulates in the hood provided at the top of the reactor. It is then conveyed through piping (Figure 3) to the biogas header (Figure 4) through a foam trap (Figure 5) and a sediment trap. In these traps the gas is made to bubble through water separating the foam and entrainments. The gas is led to a header from where it is taken to various utilities such as gas burner for food preparation or in the boiler for steam generation. A flame arrestor is installed before gas enters the utilities as a safety precaution.

It has been reported that there is saving of around Rs 64,000 per month by the use of biogas generated (Rs 800 cu m/day) from a reactor of 600 cu m capacity in a product dairy processing two lakh litres of milk per day. This saving is from eliminating the use of LPG cylinders used in canteen and workers quarters (gas generated during day is entirely used up for this purpose) as well as savings of wood in wood fired boiler of 4 TPH capacity estimated to be 0.50 MT/day wood.

Additionally, the apparent energy savings on account of elimination of motors for primary treatment reported to be Rs 76,000 per month. Thus the net saving works out to Rs 1,40,000 per month and Rs 16,80,000 per annum giving a payback period of about four years. 

UASB can be effectively utilised for primary treatment of dairy effluent. Apart from energy savings and drastic reduction of BOD/COD load, the biogas generated can be effectively utilised for various heating applications including food preparation and generation of steam.


Figure 3: Photograph of gas piping at reactor top


Figure 4: Photograph of gas header


Figure 5: Photograph of foam trap