Unit-5
Waste water Treatment
Q1) What are the unit operations that are involved in the process of waste water treatment?
A1)
Municipal wastewater treatment typically comprises a string of physical, chemical, and biological processes aimed at the removal of the polluting load and the production of a final product that can safely be disposed of in watercourses and/or reused.
Of the various processes involved, physical processes, which are also commonly referred to as unit operations, play a major role throughout the various treatment stages. In a conventional wastewater treatment plant (WWTP), one can easily identify several such stages and typical efficiencies in terms of biochemical oxygen demand (BOD) and suspended solids (SS) removal.
1. Preliminary treatment (also referred to as pretreatment) - It aims to remove bulky and large solids, thus preconditioning the effluent prior to the remainder of the treatment. This stage involves several unit operations such as screening, comminuting, and sedimentation for grit removal, skimming, and flow equalization.
2. Primary treatment - Unit operations such as sedimentation and flotation are employed to remove the suspended and colloid fractions of the effluent.
3. Secondary treatment - It aims to remove organic material through biological processes.
4. Tertiary or advanced treatment - It aims to remove nutrients such as nitrogen and phosphorous, residual suspended solids, inorganic and toxic and refractory organics that have escaped from previous stages. This can be done through physical (e.g., filtration, screening, air stripping, adsorption, ion exchange), chemical (precipitation, oxidation), or biological processes. However, with the exception of very few countries that have nutrient removal regulations for ecological reasons, typical WWTPs do not go beyond secondary treatment.
5. Disinfection - Disinfection removes water pathogens by chemical (e.g., chlorination or ozonation) or physical processes (e.g., UV irradiation). The treated effluent can then be safely discharged in natural receivers or partially reused (e.g., for irrigation).
6. Sludge treatment - Sludge treatment includes processes such as thickening, dewatering, drying, and digestion and aims to reduce the volume of the sludge to be handled as well as to stabilize biologically the final product, which is usually then sent to landfills (although it also may be used as fertilizer or fuel).
Q2) Which difficulties we do come across during the operation of a tricking filter? How they can be controlled?
A2)
Trickling filter tanks are generally constructed above the ground. They may either be rectangular or more generally circular. Rectangular filters are provided with a network of pipes having fixed nozzles, which sprays the incoming sewage into air, which then falls over the bed of the filter under gravity. While the circular filter tanks are provided with rotary distributors having a number of distributing arms (generally four arms are used). These distributors rotate around a central support either by an electric motor or more generally by the force of reaction on the sprays. The rate of revolutions varies from 2 RPM for small distributors to less than ½ RPM per large distributors. The distributing arms should remain about 15-20 cm above the top surface of the filtering media in the tank.
There is an important difference between the action of rotary distributors and that of spray nozzles. With a rotary distributor, the application of sewage to filter is practically continuous whereas with spray nozzles, the filter is closed for 3-5 minutes and then rested for 5-10 minutes before the next application.
The closing tank for a filter with circular distributors will, however, be designed to have a smaller capacity (about 1- 3 minutes detention capacity), as against a higher capacity (about 5-10 minutes detention capacity) for filters with spray nozzles.
The filtering media consists of coarser materials like cubically broken stones or slag, free from dust and small pieces. The size of the material used may vary between 25-75 mm. The depth of filtering media may be between 2-3 metres. The filtering material may be placed in layers, with coarsest stones used near the bottom and finer materials towards the top.
The walls of filter tanks are made honey combed or otherwise provided with openings for circulation of air. Satisfactory ventilation is achieved when properly designed under-drains having adequate openings are provided under the filter bed. Besides ensuring satisfactory drainage, such drains, will also ensure satisfactory ventilation and aeration of the filter bed. Vitrified clay blocks are generally used as under drains which have top openings of such size that the stone can be placed directly on them. These blocks are laid on a reinforced concrete floor (about 10-15 cm thick) which is sloped gently towards the main effluent rectangular channel. This main effluent channel may be provided adjoining the central column of the distributor or may be provided along the circular periphery of the filter. The slope of the channel should be sufficient to ensure a flow velocity of about 0.9 m/sec. The depth and width of this central channel should be such that maximum flow is carried below the level of the under drains.
Q3) Design a rapid sand filter to treat 10 million litres of raw water per day allowing 0.5% of filtered water for backwashing. Half hour per day is used for backwashing. Assume necessary data.
A3) Total filtered water = 10.05 x 24 x 106/24 x 23.5 = 0.42766 Ml / h
Let the rate of filtration be 5000 l / h / m2 of bed.
Area of filter = 10.05 x 106 x1 /23.5 X 5000 = 85.5 m2
Provide two units.
Each bed area 85.5/2 = 42.77. L/B = 1.3; 1.3B2 = 42.77
B = 5.75 m; L = 5.75 x 1.3 = 7.5 m
Assume depth of sand = 50 to 75 cm.
Under drainage system:
Total area of holes = 0.2 to 0.5% of bed area.
Assume 0.2% of bed area = 0.2 x 42.77/100 = 0.086 m2
Area of lateral = 2 (Area of holes of lateral)
Area of manifold = 2 (Area of laterals)
So, area of manifold = 4 x area of holes = 4 x 0.086 = 0.344 = 0.35 m2
\ Diameter of manifold = (4 x 0.35 /p) X 1/2 = 66 cm
Assume c/c of lateral = 30 cm. Total numbers = 7.5/ 0.3 = 25 on either side.
Length of lateral = 5.75/2 - 0.66/2 = 2.545 m.
C.S. area of lateral = 2 x area of perforations per lateral. Take dia of holes = 13 mm
Number of holes: n p (1.3)2/4 = 0.086 x 104 = 860 cm2
\ n = 4 x 860 / p (1.3)2= 648, say 650
Number of holes per lateral = 650/50 = 13
Area of perforations per lateral = 13 x p (1.3)2 /4 = 17.24 cm2
Spacing of holes = 2.545/13 = 19.5 cm.
C.S. area of lateral = 2 x area of perforations per lateral = 2 x 17.24 = 34.5 cm2
\ Diameter of lateral = (4 x 34.5/p)1/2 = 6.63 cm
Check: Length of lateral < 60 d = 60 x 6.63 = 3.98 m. l = 2.545 m (Hence acceptable).
Rising wash water velocity in bed = 50 cm/min.
Wash water discharge per bed = (0.5/60) x 5.75 x 7.5 = 0.36 m3/s.
Velocity of flow through lateral = 0.36 = 0.36 x 10 X 4 = 2.08 m/s (ok)
Total lateral area = 50 x 34.5
Manifold velocity = 0.36/0.345=1.04 m/s < 2.25 m/s (ok)
Wash water gutter
Discharge of wash water per bed = 0.36 m3/s.
Size of bed = 7.5 x 5.75 m.
Assume 3 troughs running lengthwise at 5.75/3 = 1.9 m c/c.
Discharge of each trough = Q/3 = 0.36/3 = 0.12 m3/s.
Q =1.71 x b x h3/2
Assume b =0.3 m
h3/2 = 0.12 = 0.234
1.71 x 0.3
\ h = 0.378 m = 37.8 cm = 40 cm
= 40 + (free board) 5 cm = 45 cm; slope 1 in 40
Clear water reservoir for backwashing
For 4 h filter capacity, Capacity of tank = 4 x 5000 x 7.5 x 5.75 x 2 /1000 = 1725 m3
Assume depth d = 5 m. Surface area = 1725/5 = 345 m2
L/B = 2; 2B2 = 345; B = 13 m & L = 26 m.
Dia of inlet pipe coming from two filter = 50 cm.
Velocity <0.6 m/s. Diameter of wash water pipe to overhead tank = 67.5 cm.
Air compressor unit = 1000 of air/ min/ m2 bed area.
For 5 min, air required = 1000 x 5 x 7.5 x 5.77 x 2 = 4.32 m3 of air.
Q4) Write a note on different modes of aeration in activated sludge process.
A4)
Basically, there are two methods of introducing air into the aeration tanks i.e.
1. Diffused air aeration or air diffusion
In this method, compressed air under a pressure of 0.35 to 0.7 kg/cm2 is introduced into the aeration chambers, through diffusion plates or other devices, called diffusers- porous plates and porous tubes made up of quartz or crystalline alumina and capable of diffusing air in small bubbles, so as to provide the greatest possible efficiency of aeration. Plates are generally square in shape with dimensions of 30 cm × 30 cm and are usually 25 mm thick which are fixed at the bottom of aeration tanks. Tube diffusers are generally 60 cm long with internal diameter of 75 mm and thickness of wall equal to 15 mm. These tubes are suspended in the aeration tank and can be taken out for cleaning, without emptying the tank. This type of aeration is achieved in different types of aeration tanks as under:
2. Mechanical aeration
In the air diffusion method, a lot of compressed air (90-95%) gets wasted, as it simply escapes through the tank without giving oxygen to the sewage, although it helps in bringing about required agitation of sewage mixture. To combat this problem, in mechanical aeration method, atmospheric air is brought in contact with the sewage and sewage is stirred up by means of mechanical devices like paddles to introduce air into it from the atmosphere by continuously changing the surface of sewage by circulation of sewage from bottom to top. Aeration period depends on the mechanical process adopted for agitation and it generally varies between 6-8 hours. Two types of aerators are used in this type of aeration e.g.
3. Combined diffused and mechanical aeration
In this method, the diffused air aeration as well as mechanical aeration is combined together in a single unit. A well known type of such an aerator unit is called Dorroco aerator. In this type of aerator, the aeration of sewage is achieved by diffusing air through bottom diffuser plates as well as by rotating paddles at the rate of 10-12 rpm. Spiral motion, so set up, brings about the required aeration.
Such an aerator is very efficient; detention period is smaller (3-4 hours) and requires less amount of compressed air as compared to the diffused air aeration.
Q5) What is the difference between trickling filter and Rotating Biological Contractors (RBCs)?
A5)
Trickling Filter
These filters are also known as sprinkling or percolating filters. The conventional trickling filters and their improved form known as high-rate trickling filters are now almost universally adopted for giving secondary treatment to sewage.
Trickling filter is a bed of crushed stone, gravel or slag of relatively large size to which the settled sewage is applied by sprinkling on the surface. The applied sewage trickles in a thin film over the surface of filtering media which have become coated with a zoogloeal film. This zoogloeal film includes zoogloeal forming and other bacteria, fungi, protozoa and algae. Both suspended and dissolved solids come in contact with this film. As the condition is aerobic, a large number of aerobic bacteria will inhabit the film, reacts with the organic solids either in suspended, colloidal or dissolved form. This brings about the reduction of biological oxygen demand (BOD), ammonia, organic nitrogen etc. In order to ensure the large .scale growth of the aerobic bacteria sufficient quantity of oxygen is supplied by providing suitable ventilation facilities in the body of the filter and also to some extent by the intermittent functioning of the filters. The contact between filter media and wastewater is allowed only for a short time. The bed is then drained and allowed to rest before the next cycle is repeated. A typical cycle requires 12 hours (6 hours for operation and 6 hours for resting). Temperature will affect the efficiency of tickling filters. In warm climate, the efficiency of BOD removal by trickling filters is higher.
Rotating Biological Contractors
The rotating biological contractor's process of secondary waste water treatment has been recently developed and does not fit precisely into either the trickling filter or the activated sludge categories: but does employ principle common to both of them.
A rotating biological contractor (RBC) is a cylindrical media made of closely mounted thin flat circular-plastic sheets or discs of 3-35 m in diameter, 10 mm thick and placed at 30-40 mm spacing mounted on a common shaft.
The RBCs are usually made in up to 8 m in length and may be placed in series or parallel in a specially constructed tank through which the wastewater is allowed to pass. The RBCs are kept immense in wastewater by about 40% of their diameter. They are rotated around their central horizontal shaft, at a speed of 1-2 rpm by means of power supplied to the shaft. Approximately 95% of the surface area is thus alternatively immersed in the wastewater and then exposed to the atmosphere above the liquid.
When the process is operated, the micro-organisms of the wastewater begin to adhere to the rotating surfaces and grow there until the entire surface area of the discs gets covered with 1-3 mm thick layer of biological slime. As the discs rotate, they carry a film of wastewater into the air, where it trickles down the surface of the discs absorbing oxygen. As the discs complete their rotation, this film mixes with the wastewater in the tank, adding to the oxygen of the tank and mixing the treated and partially treated waste water. As the attached microorganisms pass through the tank, they absorb other organics for breakdown. The excess growth of micro-organisms is sheared from the discs, as they move through the wastewater tank. The dislodged organisms are kept in suspension by the moving discs. This suspended growth finally moves down with the sewage flowing through the tank to a downstream settling tank for removal.
The effluent obtained from this process is of equal or even better quality than obtained from other secondary treatments. The quality of effluent can further be improved by placing several contractors in series along the tank.
Q6) Design a low-rate filter to treat 6.0 Mld of sewage of BOD of 210 mg/l. The final effluent should be 30 mg/l and organic loading rate is 320 g/m3/d.
A6)
Assume 30% of BOD load removed in primary sedimentation i.e., = 210 x 0.30 = 63 mg/l.
Remaining BOD = 210 - 63 = 147 mg/l
Percent of BOD removal required = (147-30) x 100/147 = 80%
BOD load applied to the filter = flow x conc. of sewage (kg/d) = 6 x 106 x 147/106 = 882 kg/d
To find out filter volume, using NRC equation
E2 =100 / 1+0.44(F1.BOD/V1.Rf1)1/2
80 = 100 / 1+0.44(882/V1)1/2 Rf1= 1, because no circulation.
V1= 2704 m3
Depth of filter = 1.5 m, Fiter area = 2704/1.5 = 1802.66 m2, and Diameter = 48 m < 60 m
Hydraulic loading rate = 6 x 106/103 x 1/1802.66 = 3.33m3/d/m2< 4 hence o.k.
Organic loading rate = 882 x 1000 / 2704 = 326.18 g/d/m3 which is approx. equal to 320.
Q7) Briefly explain low-rate anaerobic digesters tank.
A7)
Anaerobic digestion has become an increasingly popular technology for treating wastewater. We specialize in design and construction of anaerobic wastewater treatment plants on turnkey basis. We have installed more than 60 low rate anaerobic digester systems for various industries such as distilleries, breweries, pharmaceuticals, pulp & paper, citric acid, yeast, chemicals, etc. These digesters provide a large volume (hydraulic retention time: 15- 45 days), offering better process stability with consistent performance and biogas generation. BOD reductions of 80%- 95% and COD reductions 60% – 80% can be achieved, depending on the nature of the wastewater. These digesters can be constructed in a rectangular or circular above ground or below ground construction.
In absence of oxygen i.e. under anaerobic environment, bacteria decompose organic pollutants (BOD, COD, etc.) to carbon dioxide, methane and water. The biogas here is generated through a series of biomethanisation reactions. The large quantities of biogas generated have been used to replace fuel in boilers and to generate electricity in generators, offering quick returns to our clients.
Benefits of this anaerobic system includes high reductions in organic load and thus less loading and power consumption in the downstream treatment process. In addition, the use of biogas as a renewable non-conventional source of energy is possible.
Due to the environmental benefits of anaerobic digestion, we have been able to secure grants and funding for some of our anaerobic digestion projects. These grants and funding opportunities can offset the cost of the anaerobic treatment system, further improving the economics of using anaerobic treatment. Additional streams of revenue in the form of carbon credits can also be secured which can allow for a design-build-finance-operate form of project delivery – We recently executed a project that was delivered in this manner.
Q8) Briefly explain high-rate anaerobic digesters tank.
A8)
The static granular bed reactor (SGBR) was developed by researchers at Iowa State University. The SGBR system utilizes anaerobic granules like the UASB reactor to treat wastewater. Unlike other granular reactors the SGBR does not require mixers, gas-liquid-solid separation devices, recirculation pumps, or heat exchangers. The SGBR has a simple down flow configuration, allowing influent to flow through a bed of active anaerobic granules. The downward flow regime of the SGBR allows for the biogas to be easily separated from the granule bed and liquid at the top of the system. The SGBR uses active granules similar to the UASB reactor, but it operates in a down flow configuration instead of an up flow. Due to the down flow configuration, the SGBR acts like a bioreactor and a filter and are not susceptible to solids washout under high hydraulic loading rates like the UASB.
The SGBR has been used successfully to treat a variety of wastewaters including synthetic wastewater consisting of non-fat dry milk, industrial wastewater, pork slaughterhouse wastewater, landfill leachate, and dairy wastewater. The results obtained from using the SGBR system to treat municipal wastewater are displayed in. At steady state operation the SGBR system had COD removal efficiencies between 74 and 84%. Conversely to the UASB reactor, the SGBR’s ability to remove TSS increased when the HRT of the system was decreased. As the hydraulic flow into the system increases, the granule bed of the SGBR may become more compact. This bed compaction and decrease in the bed porosity of the SGBR system improves solid entrapment and retention.
Residuals from primary wastewater treatment are a combination of floating fats, oils and grease along with solids collected at the bottom of the primary clarifier. The residuals from secondary wastewater treatment are composed primarily of microbial cells (proteins and polysaccharides) and suspended solids produced during aerobic biological treatment. The mixture of primary and secondary sludge is composed of 60 to 80% carbohydrates, fats and proteins. Approximately 80% (30% primary sludge and 50% secondary sludge) of the organic waste input into a WWTP ends up in the anaerobic digesters. While, estimates that 40 to 60% of the total organic matter in raw sewage is collected from primary and secondary clarifiers and sent to the anaerobic digesters.
Q9) What are the steps that are involved in the design of the septic tank?
A9)
The capacity of septic tank depends on number of users and interval of sludge removal. Normally sludge should be removed every 2 years. The liquid capacity of tank is taken as 130 liters to 70 liters per head. For small number of users 130ltr per head is taken.
A septic tank is usually provided with brick wall in which cement mortar [not less than 20cm (9 inch)] thick and the foundation floor is of cement concrete 1:2:4. Both inside and outside faces of the wall and top of the floor are plastered with minimum thickness of 12mm (one-half inch) thick cement mortar 1:3 mix.
Dimensions of Septic Tank Components
i) Length, Width and Depth of Septic Tank
Width = 750mm (min)
Length = 2 to 4 times width
Depth = 1000 to 1300mm. (min below water level) + 300 to 450mm free board
Maximum depth = 1800mm + 450 mm free board
Capacity = 1 cubic meter (10 cubic feet) minimum
ii) Detention period
Detention period of 24hrs (mostly) considered in septic tank design. The rate of flow of effluent must be equal to the rate of flow of influent.
iii) Inlet and outlet pipes
An elbow or T pipe of 100mm diameter is submerged to a depth of 250-600mm below the liquid level. For outlet pipe an elbow or T type of 100mm diameter pipe is submerged to a depth of 200-500mm below the liquid level. Pipes may be of stone ware or asbestos.
iv) Baffle Walls of Septic Tank
For small tanks, RCC hanging type scum baffle walls are provided in septic tanks. Baffle walls are provided near the inlet. It is optional near the outlet.
The inlet baffle wall is placed at a distance of L/5 from the wall, where L is the length of the wall. The baffle wall is generally extended 150mm above to scum level and 400-700mm below it.
v) Roofing Slab of Septic Tank
The top of the septic tank is covered with a RCC slab of thickness of 75-100mm depending upon the size of the tank. Circular manholes of 500mm clear diameter are provided for inspection and desludging. In case of rectangular opening clear size is kept as 600X450mm.
vi) Ventilation Pipe
For outlet of foul gases and ventilation purpose cast iron or asbestos pipe of 50-100mm diameter is provided which should extend 2m (min) above ground level. Top of the ventilation pipe is provided with a mosquito proof wire mess or cowl.
Q10) What are the different advantages and disadvantages in activated sludge process?
A10)
Advantages of an activated sludge process
Disadvantages of activated sludge process
