UASB Units

UASB Units UASB type units are one in which no special media have to be used since the sludge granules themselves act as the 'media' and stay in suspension. A typical arrangement of a UASB type treatment plant for municipal sewage would be as follows:Initial pumping Screening and degritting Main UASB reactor Gas collection and conversion or conveyance Sludge drying bed Post treatment facility

In the UASB process, the whole waste is passed through the anaerobic reactor in an upflow mode, with a hydraulic retention time (HRT) of only about 8-10 hours at average flow. No prior sedimentation is required. The anaerobic unit does not need to be filled with stones or any other media; the upflowing sewage itself forms millions of small "granules" or particles of sludge which are held in suspension and provide a large surface area on which organic matter can attach and undergo biodegradation.

A high solid retention time (SRT) of 30-50 or more days occurs within the unit. No mixers or aerators are required. The gas produced can be collected and used if desired.

Anaerobic systems function satisfactorily when temperatures inside the reactor are above 18-20C. Excess sludge is removed from time to time through a separate pipe and sent to a simple sand bed for drying.

In all biological waste treatment processes some surplus sludge is produced. The objective of residual management is:Reduction of water content. Stabilization of sludge solids. Reduction in sludge solids volume. In facultative type aerated lagoons and algal waste stabilization ponds, the surplus sludge settles out in the unit itself and is removed only once in a few years after emptying the unit, exposing the wet sludge to natural drying, and carting away the dried sludge for agricultural use or land filling.In extended aeration process where aerobic digestion of surplus sludge is done, the sludge can be taken directly for dewatering and disposal. In case of activated sludge and trickling filter plants, the sludge is taken (along with the primary sludge) to a sludge digester for further demineralization and thereafter it is dewatered.

Residual Management

Natural: sludge drying beds, sludge lagoons Mechanical: sludge thickeners, centrifuges, vacuum filters, filter press Physical: heat drying, incineration Disposal of Sludge Final disposal of sludge is to land and sometimes to the sea, in one of the following ways:Agricultural use of dried or wet sludge. Use of dried sludge as landfill in absence of agricultural demand. Spreading wet sludge on eroded or waste land, contouring the field, so as to gradually build up a top soil of agricultural value. Disposing off wet sludge along with solid wastes for (i) composting, or (ii) sanitary landfill. Transporting and dumping into the sea.

Sludge Dewatering MethodsSand beds are generally constructed as shown in the typical cross-sectional view.Sludge is generally spread over the sand which is supported on a gravel bed, through which is laid an open-joint earthen pipe 15 cm in diameter spaced about 3 m apart and sloping at a gradient of 1 in 150 towards the filtrate sump. The drying beds are often subdivided into smaller units, each bed 5-8 m wide and 15-50 m long. The drying time averages about 1-2 weeks in warmer climates, and 3-6 or even more in unfavourable ones.

Sand Beds for Sludge Drying

The stabilization ponds are open flow through basins specifically designed and constructed to treat sewage and biodegradable industrial wastes. They provide long detention periods extending from a few to several days. Pond systems, in which oxygen is provided through mechanical aeration rather than algal photosynthesis are called aerated lagoons. Lightly loaded ponds used as tertiary step in waste treatment for polishing of secondary effluents and removal of bacteria are called maturation ponds.

Stabilization Ponds

Aerobic ponds are shallow ponds with depth less than 0.5 m and BOD loading of 40-120 kg/ha.d so as to maximize penetration of light throughout the liquid depth. Such ponds develop intense algal growth. Anaerobic ponds are used as pretreatment of high strength wastes with BOD load of 400-3000 kg/ha.d Such ponds are constructed with a depth of 2.5-5m as light penetration is unimportant. Facultative pond functions aerobically at the surface while anaerobic conditions prevail at the bottom.They are often about 1 to 2 m in depth. The aerobic layer acts as a good check against odour evolution from the pond.

Classification of Stabilization Ponds

The functioning of a facultative stabilization pond and symbiotic relationship in the pond are shown below. Sewage organics are stabilized by both aerobic and anaerobic reactions. In the top aerobic layer, where oxygen is supplied through algal photosynthesis, the non-settleable and dissolved organic matter is oxidized to CO2 and water. In addition, some of the end products of partial anaerobic decomposition such as volatile acids and alcohols, which may permeate to upper layers are also oxidized periodically. The settled sludge mass originating from raw waste and microbial synthesis in the aerobic layer and dissolved and suspended organics in the lower layers undergo stabilization through conversion to methane which escapes the pond in form of bubbles.

Mechanism of Purification

Oxidation ditchThere are two general methods of disposing of the sewageDilution ( disposal in water)Sewage farming( disposal on land)Unit V Disposing of the sewage effluents and sludgeDilutionWhen wastewater is discharged into the receiving water, dilution takes place due to which the concentration of organic matter is reduced and the potential nuisance of sewage is also reduced. When the dilution ratio is quite high, large quantities of DO are available which will accelerate the chances of purification and reduce pollution effects.Aerobic condition will always exist because of higher dilution. This will however, not be there if dilution ratio is small, i.e., when large quantities of oxygen demanding effluent is discharged into a small stream supplementing little oxygen or aeration.

Actions Involved in Self-purification river

Self-purification of stream largely depends upon currents, which will readily disperse the wastewater in the stream, preventing local accumulation of pollutants.High velocity accelerates reaeration and reduces the concentration of pollutants. High velocity improves reaeration, reduces the time of recovery, though length of stream affected by the wastewater is increased.

2.Dispersion due to Currents

If the stream velocity is lesser than the scour velocity of particles,sedimentation will take place, which will have two effects.(i) The suspended solids, which contribute largely the oxygen demand, will be removed by settling and hence water quality of the downstream is improved.(ii) Due to settled solids, Anaerobic decomposition may take place.

3. Sedimentation

At low temperature, the activities of bacteria is low and hence rate of decomposition will also be slow, though DO will be more because of increased solubility of oxygen in water. At high temperatures, the self-purification takes lesser time, though the quantity of DO will be less.

4. TemperatureSunlight helps photosynthesis of certain aquatic plants (as algae) to absorb carbon dioxide and give out oxygen, thus accelerating self-purification. Sunlight acts as a disinfectant5. SunlightThe Self-purification process of a stream polluted by the wastewater or effluent discharged into it can be divided into the following four zones:(i) Zone of Degradation (Decomposition zone)(ii) Zone of Active Decomposition (Septic zone)(iii) Zone of Recovery(iv) Zone of Clear Water

Zones of pollution in streams (Oxygen sag analysis) This zone is situated just below the outfall sewer while discharging itsContents into the stream. In this zone, water is rendered dark and turbid, having the formation of sludge deposits at the bottom. The DO is reduced to 40% of the saturation values. There is an increase in CO2 content, and reaeration is much slower than deoxygenation. (Though conditions are unfavourable for aquatic life, fungi at shallow depths and bacteria at greater depths breedAlong with small worms, which work over and stabilize the sewage and sludge). The decomposition of solid matter takes place in this zone and anaerobic decomposition prevails over aerobic decomposition.

Zone of DegradationThis zone is just the continuation of degradation zone and is marked by heavy pollution. Water in this zone becomes grayish and darker than the previous zone. The DO concentration in this zone falls down to zero. Active anaerobic organic decomposition takes place, with the evolution of Methane (CH4), Hydrogen sulfide (H2S), Carbon dioxide (CO2) and Nitrogen (N2) bubbling to the surface with masses of sludge forming black scum. Fish life is absent in this zone but bacterial flora will flourish with the presence of anaerobic bacteria at upper end and aerobic bacteria at the lower end. However, near the end of this zone, as the decomposition slackens, reaeration sets in and DO again rises to its original level of 40% (of saturation value).

Zone of Active DecompositionIn this zone, the process of recovery starts, from its degraded condition to its former purer condition. The stabilization of organic matter takes place in this zone. Due to this, most of the stabilized organic matter settles as sludge, BOD falls and DO content rises above 90% value. Near the end of the zone, fungi wave out and algae reappear.

Zone of Recovery In this zone, the natural condition of stream is restored with the result that(i) Water becomes clearer and attractive in appearance.(ii) DO rises to the saturation level, and BOD drops to the lowest value.(iii) Oxygen balance is attained.

Clear Water Zone