improved bio-tower technologyhnbc.in/pdf/sewage-treatment/bio-tower/bio-tower-technical-note.pdf ·...

IMPROVED BIO-TOWER TECHNOLOGY FOR

MUNICIPAL SEWAGE TREATMENT

Regd. Office : ‘Henabh Center’, 1326, Shukrawar Peth, Off. Bajirao Road, PUNE : 411 002 (INDIA),

Tel. : 24473299 (5 Lines) 24474696, (Voice Mail) Fax : 0091‐20‐24473185. E‐mail : [email protected] Web Site : www.hnbc.in

IMPROVED BIO‐TOWER TECHNOLOGY FOR MUNICIPAL SEWAGE TREATMENT 1

IMPROVED BIO-TOWER TECHNOLOGY FOR MUNICIPAL SEWAGE TREATMENT

The treatment scheme consists of A) Primary Treatment Units, B) Secondary Treatment Units, C) Solid liquid separation Units and D) Disinfection Arrangement.

PROCESS DIAGRAM

Stilling Chamber Improved Bio-Towers Chlorine Fine screen Channel Aeration tanks Contact

Grit Chamber Secondary Settling Tank Parshall Flume Distribution Chamber

Primary Treatment Units consist of Stilling Chamber, Fine Screen Channel, Grit Chamber, Flow measurement arrangement and Distribution Chamber Stilling Chamber reduces the velocity of the incoming sewage before it enters the fine screen channel. Fine Screen Channel is designed to remove the particles of size more than 5 mm. Drum type fine screens are proposed in the plant. Separated solids will be taken on conveyor belt and collected at bottom through chute. Grit Chamber is designed to remove inorganic matter of specific gravity more than 2.5 and size more than 0.15 mm. The settled grit is removed from the chamber by grit mechanism to the side channel. Grit is removed by grit screw from the channel and taken to bottom through chute. While doing this grit is washed with the help of grit wash pumps to separate organic matter attached to grit if any. After grit chamber flow is measured and taken to distribution chamber to distribute the flow to the Improved Bio-Towers. Secondary Treatment Units consist of Improved Bio-Tower, Mixing Chamber and Aeration Tank.

PRIMARY TREATMENT

SECONDARY TREATMENT

DISINFECTION TREATMENT

SLUDGE HANDELING SYSTEM


TRICKLING FILTERS with rock packing have been a common simple and low energy process used for secondary treatment since early 1900s. The concept of a trickling filter grew from the use of contact filters in England in the late 1890s. Biological growth on the media surface converts the organic waste material in to stable by-products such as CO2, H2O using microorganisms mainly the bacteria. The old plants used mainly Rock as the surface for the attached growth biological systems. This media had disadvantages due to heavy weight, elaborate supporting structure, water tight peripheral walls designed to take lateral thrust, low voltage, potentials for plugging, fly nuisance, poor wetting characteristics, etc. Even though the problems with rock media were leading and increased use of suspended growth biological systems, the advantageous of the process as such prevented the systems from going in to oblivion. Advantages of Attached Growth Fixed Film Bio-Filters Over Suspended Growth Activated Sludge Process Plants for Waste Water Treatment

Attached growth systems are more suitable to take shock loads in terms of hydraulic and organic disturbances as compared to suspended growth systems.

Very high capacity to take organic load. Trouble free operations even under difficult conditions. Reliable and consistent nitrification, both in summer & winter operations. Low power consumption - to the tune of up to half of conventional ASP and 1/4th of

Extended Aeration plant. Very low skilled operation, control and maintenance requirements and thus low

operation and maintenance costs. Low sludge production and easily drainable sludge. 8 .Much smaller secondary settling tanks due to higher settling rates of the sludge

produced. Lower residual suspended solids concentration in treated wastewater. Cost effective combinations with other treatment methods. Improved plastic media design have significantly reduced tower sizes and costs and

enhanced their capacities and efficiencies. Installation depths up to 6.0 m require no intermediate supports. Higher installation

depths with intermediate supports can be provided. This combined with efficient geometry can offer a very compact plant.


General Advantages of Improved Bio-Tower Application:

Long life of over 30 years - UV stabilized material. Large specific surface areas to accommodate large microbial growth. Very large voids from 90 to 97% Very weight easy to handle and simpler for supporting structure. The distribution pattern provides utilization of the complete surface area. Large depth - say 6 to 8 m., allows considerable saving of land. Due to cross flow configuration. The liquid has about 4 times higher detention time. 8 It has very low resistance to airflow leading to maximum contact of air, waste

water & bio mass. Low resistances to airflow and the mixing caused at the cross over points in the fill

lead to optimum aeration. Good wetting, continuous air movement and thinner biological growths eliminate fly

nuisance. Due to continuous redistribution of water flow, plastic media has a very high

resistance to plugging. Settled effluents TSS is much lower with Improved Bio-Tower. No elaborate peripheral walls required because modules are self-supporting, only

cladding suffices. Requires no elaborate under drain system.

AERATION BY NATURAL DRAUGHT Plastic Media have increased the BOD5 removal levels 90% apart from increasing the organic and hydraulic Loading rates. The plastic media offer very low resistance to air, provides Maximum contact of air, waste & bio mass, provide thousands of crossover points per m3, thus leading to a very good aeration by natural draught or ventilation. The air requirement is met by providing Ventilation openings in the wall, at the bottom of the Improved Bio-Tower, having a size of 4- 6% of the cross-sectional area of the Bio-Filter. This way a daily air volume of more than 100 m3 air per m3 of fill circulates to provide the necessary oxygen to the microorganisms.


The Oxidation of Organic matter in waste water and synthesis of new microbial cells takes place in following way

Oxidation COHNS + O2 ------------- CO2 + NH3 +C5H7NO2 + Other end products Organic matter Bacteria New cells

Endogenous respiration C5H7NO2 + 5O2 ---------- 5CO2 + 2H2O + NH3 + energy New cells Bacteria

SIMPLE SUPPORT SYSTEM The modules on the bottom layer are installed with the longer sides at right angles to the support beams. The module supports can be nominal 100 to 150 mm beams at 600 mm or 900 mm centers. This is possible due to the strength of the module that requires supports at only 2 points and it’s lightweight. Such simple supports, widely spaced, maximize the flow of air and water through the media and the plenum area, with least obstruction. The support system is designed

for about 400 kg/m2 per M of media depth. Below the support beams a base slab, with approx. 4% slope, collects the treated liquid into a central or peripheral channel. LONGER RETENTION TIME


The 60 cross flow characteristics provide about 3 to 4 times more detention time than the other media such as rock, since no free fall of liquid can take place inside the media. Due to this configuration, the wastewater is distributed over the entire surface inside the media, resulting in a thin liquid film. This leads to sufficient time being available for the Biological process and a high reduction of BOD. LOW CONSTRUCTION COST The media is self-supporting, strong and can be cut to fit into any regular shaped Improved Bio-Tower. With no lateral thrust or pressure on the peripheral surrounding walls, these walls can be of a cheap construction and of a simple nature. Normally Exterior cladding consists of easily available AC or plastic sheets. DISTRIBUTION SYSTEM

The system of interconnected channels in the plastic media makes it possible to use either a rotary distributor or a fixed nozzle system to achieve the desired wetting. The fixed nozzle system can be adapted to any regular shaped media plan including the circular trickling filters and is ideally suited for the rectangular or square tanks. The efficiency is independent of the distribution system at the top since the internal redistribution system in the module

starts to operate immediately on the wastewater hitting the top layer. In over 6000 mixing points per m3, the liquid film is broken up and re-mixed, a turbulence of vital importance for efficient transfer of oxygen and the dissolved organic matter between the liquid and the bio mass. DESIGN OF IMPROVED BIO-TOWERS In biological treatment of waste waters many parameters such as the BOD and COD levels, origin, waste and ambient temperatures, pH, nutrient balance, presence of toxic matters, etc. interact in such a way that each plant be designed individually. Domestic sewage usually contains all the necessary nutrients and trace minerals for biological action within the tower and the design of Improved Bio-Towers for its secondary treatment differs to a certain extent from the normal design of a high rate unit. The design of a Improved Bio-Tower is based on various design equations such as the modified VELZ equation for which the values of the constants have been experimentally


determined and fixed for these media. Further, based on the empirical data collected over the years, design curves have been developed for the organic loading for different ambient temperature ranges. Since it is also necessary to have a minimum amount of wastewater to utilize fully all the available surface area within a Bio-filter, design curves have also been developed for the hydraulic load or the wetting rates for different distribution systems and the selected depths of media. Due to the very high efficiency of these filters, no recirculation is normally necessary. However, recirculation of the Bio-filter effluent must be undertaken to satisfy the wetting rate recommendations if they are not otherwise. THE MODULE The modular medium for high rate Improved Bio-Tower is manufactured from plastic/ rigid PVC, Which is chemically resistant to water soluble substances occurring in municipal and industrial waste waters. It is also resistant to rot, bacterial, fungal or growths of other microorganisms. PROPERTIES OF PLASTIC MEDIA:

Specific Surface Area (sq m/cum)

: 90 to 200 Void ratio

: 90 to 97 % Number of mixing points per cum

: Up to 10000 Sheet thickness (mm)

: 0.3 to 1.0 Weight kg/cum

Dry : 30 to 45 Wet (operational) : 350 to 650

WINTER TIME OPERATION AND ICE FORMATION: It is known that fixed film or attached growth processes are more stable at lower temperatures due to the various advantages that this system offers in terms of the conducive conditions for the required microbial growth. Apart from these advantages, the high void ratio and geometry of plastic media are key factors is increasing efficiency and minimizing ice formations. Rock and other media present large, horizontal surfaces where the falling liquid can splash back and form ice coatings on adjacent surfaces. It is on record, that even in subzero cold spells no ice formation on the Improved Bio-Tower surface is observed, even when the adjoining walls and area are under a blanket of snow.


NITRIFICATION:

Cross flow media Improved Bio-Towers with a more compact array of sheets can be used to achieve consistent, high level nitrification (> 90%conversion) when operating at a low BOD waste stream (up to 30 mg/l) containing NH3-N concentrations up to 25 mg/1. This is a highly efficient and cost effective application for which separate design data and curves have been developed. Final effluent NH3-N concentrations in the range of 0.5-2.0 mg/1 can; easily be obtained. Nitrification takes place in following way

2NH4

+ + 3O2 -------------------- 2NO2 - + 4H+ + 2H2O

Nitroso-bacteria 2NO2

- + O2 -------------------- 2NO3 Nitro-bacter IMPROVED BIO-TOWER:

As described earlier after primary treatment raw sewage is taken to Improved BIO-TOWER, where sewage is spread on packed bed of Plastic/PVC medium where fixed film of microorganism is formed over media. Biological degradation of organic matter present in sewage occurs as trickling sewage comes in contact with biological film formed over plastic media. This method is based on empirical data collected over the years, design curves have been developed for various organic loading and different temperature ranges. About 50 % organic load is removed without use of power. The reactions taking place across the fixed film or microbial layer is explained in the figure below. Mixing Chamber:

The sewage after partial treatment in Improved Bio-Tower is then taken to Mixing Chamber where recirculation sludge from secondary clarifier is mixed with partially treated sewage and the sewage is taken to Aeration tank.


Aeration Tank: About 50 % organic load is removed in Improved Bio-Tower. Further the sewage received from Improved Bio-Tower is taken to aeration tank. Air is introduced through air diffuser. Remaining organic load is reduced to achieve outlet parameter as per the norms as mentioned above. The process taking place in aeration tank is similar to that taking place in ASP as follows

The partially treated sewage from Improved Bio-Tower with about 50% of BOD will then flow to Aeration Tank. The aeration tank is provided with suitable numbers of air diffusers to supply required oxygen to develop and maintain MLSS concentration. The hydraulic retention time of this unit is maintained as per the requirement and guidelines of CPHEEO manual. This is suspended growth process and the microorganisms responsible for treatment are maintained in liquid suspension by appropriate mixing methods. In our case we are maintaining microorganisms in suspension with the help of air diffuser by providing sufficient air considering mixing requirement. The organic matter in the waste water is utilized by microorganisms partly for metabolism and partly for cell synthesis i.e. new bacteria generation. This can be explained as follows Endogenous respiration C5H7NO2 + 5O2 ---------- 5CO2 + 2H2O + NH3 + energy New cells Bacteria

Oxidation

COHNS + O2 ------------- CO2 + NH3 +C5H7NO2 + Other end products Organic matter Bacteria New cells

Proper removal of organic matter from the sewage depends on the population of microorganisms present in the aeration tank which is called MLSS or the Mixed Liquor


Suspended Solids. The MLSS in the aeration tank is maintained by recirculating part of the sludge separated in the secondary clarifier. The excess sludge generated is taken to sludge thickener for reducing its volume and for further dewatering.

Solid liquid separation units:

These are secondary clarifier and Centrifuge Unit

Secondary Clarifier:

The aerated sewage is taken to secondary clarifier. The design of settling tank is done as per CPHEEO norms .After settling supernatant is transferred for disinfection and settled sludge is partly recirculated for maintaining MLSS in aeration tank and excess is taken for thickening.

Centrifuge Unit:

The excess sludge collected from secondary clarifier is taken to sludge thickener with the help of centrifuge feed pumps the volume of sludge is reduced and sent for disposal in landfill or dumping yard.

Disinfection Arrangement:

Disinfection arrangement consists of Chlorine Contact Tank and Chlorination System

Chlorine Contact Tank:

The effluent or supernatant from Secondary Clarifier is collected in chlorine contact tank. The supernatant is disinfected in chlorine contact tank by adding suitable dose of chlorine for reducing MPN of coliforms and finally the sewage is ready for disposal.


Chlorination System: Chlorination system covering chlorine tonners, chlorinators, water feed pumps, piping, booster pumps etc. as per explosive department norms are provided in chlorination room. Advantages of “Improved Bio-Tower Technology”:

Only Column-Beam structure suffices Tower can be constructed even up to 9.0 m which saves lot of land.

Withstands shock loads.

Sludge generated in Improved Bio-Tower is highly thick and having higher density which settles faster leads to smaller clarifier size giving better effluent quality.

Sludge production is less than that produced in ASP or any other aerobic treatment method requires less sludge handling mechanism.

No problem of sludge deposition. Improved Bio-Tower is continuously aerated due to natural draft thereby avoids any odor related problems. System is always ready to receive effluent in case of interrupted power supply.

Only power requirement is to lift the effluent to the top of tower. Compared to ASP this energy saving is more than 50-60%.

Easy operation and maintenance. No foaming problems. No recirculation of sludge required. No skilled staff is required.

improved bio-tower technologyhnbc.in/pdf/sewage-treatment/bio-tower/bio-tower-technical-note.pdf ·...

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