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WASTEWATERTREATMENT PLANT DESIGN

Term Project CHE362,

Biochemical Engineering

Instructor: Dr. Sri Sivakumar

Aakash Langeh Y9004

Abhijeet Kumar Y9013

Ankesh Kumar Singh Y9090

Mitul Mundra Y9338

Shashank Y9539

Design Parameters

The overall design of the wastewater treatment plant con-sists of 4 stages: i)Primary treatment which consists ofscreening, grit removal and sedimentation ii)Secondarytreatment consists of a bioreactor iii)Tertiary treatmentconsists of nitrogen removal, adsorption and pH con-trol. For plant design, the following parameters are as-sumed/experimentally determined for the wastewater.

Type Flow BOD Solids pH aromaticL/h mg/L wt% ppm

Effluent 1000 500 5 varies 400Discharge 1000 4-5 traces 6.5 30

Further, the total flow rate and the concentrations keepvarying at different times of the day, as well as are sub-

ject to seasonal variations. For example, in morning hoursthe flow rate and BOD value is high. Similarly, during rainyseason solid contents like silt are more due to surface run-off.

Screening

Screens typically consist of wedge wire. It is carried out intwo phases. In the 1st phase also called coarse screening, thesize of the opening is 20-30 mm. It captures the large ob-

jects. In the second phase called fine screening the openingsvary between 1.5 to 6.4 mm. Cross section area of the screensis typically 1 m2. For a 1000 kg/h feed of wastewater theimpurities removed in this stage are nearly 10 kg/h. Whenthe head loss across the tank exceeds 0.6 m, the screens needto be cleaned. The waste water is pumped into the screen-ing tank using a centrifugal pump, with h= 0.15m on anaverage.

Primary Sedimentation

Sedimentation is the process of removing solid particlesheavier than water by gravity settling. In wastewater treat-

ment, sedimentation is used to remove both inorganic andorganic materials which are settleable in continuous-flowconditions. The sedimentation tank consists of a tank with2 settling funnels where solid waste settles down. Bafflesare provided to enhance the settling process. The wastesremoved in this stage 20 kg/h for a 1000 kg/h feed.Grease and oil based impurities that float on the surfaceof water during sedimentation can be recovered by saponi-fication. Skimmers are used for collecting and removingthese impurities.

Sand-bed Bioreactor

A bioreactor packed with sand of effective particle size 0.3-0.5 mm. The uniformity coefficient (a measure of similarityof shape and size in sand particles) of sand is 4. Assuming30% BOD is removed by sedimentation, about 92% BOD re-moval takes place in this reactor. 12 dosages of wastewateris given to the reactor per day, which prevents clogging, aswell as provides sufficient aeration for the micro-organisms.Since the reactor under consideration is not very deep, aera-tion may be carried out using with air daft. However, whenthe reactor is scaled up for larger flow rates, blowers may be

needed.

The recirculation is carried out in cycles using controlvalves. Initially, the entire volume in the recirculationtank is distributed onto the sand bed as a single dosage,the treated water is collected back into the recirculationtank, and 1/5th of it is discharged when the tank is full,making room for fresh wastewater. This constitutes onecycle for the reactor.

Sand and media bioreactors respond well to gradual in-creases in wastewater loading. Therefore, they are very ap-propriate for new developments with a gradual build-outrate. These bioreactors also tolerate fluctuations in flow,

especially changes from a negligible flow to very high flows.Further, the installation cost of such reactors is very low.However, these reactors cause problems of foul smell andinsects and hence need to be installed at sufficient distancesfrom residential establishments.

Nitrogen and Phosphorus removal

For nitrogen removal, a Step Feed Anoxic Aerobic Processis used. In this process, wastewater is introduced at sev-eral feed points. Phosphate release and denitrification takesplace in the anoxic zone. In the aerobic zone, nitrification

and BOD removal takes place. The effluent of the aerobicstage goes to the anoxic stage for denitrification. Phosphateremoval in the form of polyphosphate also takes place in theaerobic zone. Phosphate is removed in the form of sludge,nitrogen is released in the form of N2, and BOD is convertedto CO2 and water.Flow rate = 1000 kg/h (solute free basis)The step feed aeration tank is divided into four equal passeswith equal volumes used for the anoxic and aerobic zones.The flow split to each pass is 0.1,0.4,0.3 and 0.2 of the influ-

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S1: inlet wastewater from sewer 1m3/hr S2: froth removal S3: saponification S4: air daft S5: discharge to stream

V1: Sump tank V = 25m3 V2: Screening system with coarse and fine screensL = 10m, A= 1m2

V3: Primary sedimentation tank L = 5m V4: Recirculation tankV = 10m3 V5: Secondary sedimentation tank V = 2m3

V6: Recycle tank V = 1.6m3 V7: Adsorbent storage V8: Adsorbent recovery unit (filter)

V9: pH regulation unit V = 5m3 V10: Gas storage tankV = 0.05m3 V11: Storage of slaked lime

R1: Sand-bed bioreactor A = 130m2, h= 4.15m R2: Combined nitrogen phosphorus removal system A:anoxic B:aerobic

R3: Continuous adsorber unit with adsorbent recovery V = 1.38m3 R4: Anaerobic digestion of solid residue

C1: Controller for recirculation cycle C2: Controller for adsorbent recovery C3: Controller for pH regulation

Figure 1: Process flowsheet

ent flow, for passes 1 to 4 respectively. Recycle ratio usedis 0.6. Anoxic volume is 20% of the total reactor volume.Total volume of aeration tank is 10.5m3 (2.1m3 for anoxiczone and 8.4m3 for the aerobic zone).

Inlet OutletNH4-N-P (mg/l) 35 0.5BOD (mg/l) 30 10

Sludge Digestion

The solids sedimented out at various units may be dried

and disposed off. Or, they can be mixed with other agri-cultural wastes and added to an anaerobic sludge digestionunit. Sludge digestion involves the treatment of highly con-centrated organic wastes in the absence of oxygen by anaer-obic bacteria. The dried sludge is fed to a cylindrical RCCtank of 6m diameter and the height of sludge in the tankis 6m. The pH is maintained slightly greater than 6.0 anddigestion period is 30 days. The amount of gas producedvaries from 0.014 to 0.028 m3 per cycle consisting of 65%methane and 30% CO2 and 5% of nitrogen.The gases produced are stored in a tank, and the solidresidue is disposed or used as fertilizer.

Adsorber

To remove laboratory wastes such as heavy metals, aromat-ics, etc., a continuous adsorber is used. Activated carbon isused as the adsorbent.Solute in inlet = 400ppmAdsorbent surface area = 5m2/kgAdsorbent flow rate = 10g/l

Solute in outlet = 30ppm (within permissible limits)A controller is used that takes concentration of adsorbatein wastewater as input (measured by a sensor), and regu-lates the flow rate of adsorbent accordingly, thus reducingadsorbent requirements if the concentration of laboratorychemicals is negligible at some point in time.

pH Control Unit

The water discharged to the streams should neither behighly acidic nor alkaline. The acceptable discharge pH

6.5. The pH of water in the tank is measured, which servesas an input to a controller. If pH is not close to 6.5, theexit stream is shut off. If it is acidic, lime water from theslaked lime storage tank is mixed into it, and if alkaline, thebiogas (containing CO2) is bubbled into the tank. When pHis restored, the discharge resumes.

The treated water is discharged into natural water bodies,water harvesting tanks (to replenish ground water), or usedfor irrigation and recreational purposes.

References

1. C.C. Lee, S.D. Lin, Handbook of Environmental Engineering

Calculations, 2nd ed, McGraw Hill, 2007, Pages 1.418-1.5102. K. Mancl, J. Tao, Sand and Media Bioreactors for Wastewater

Treatment, OSU bulletin 876, 2011

3. Metcalf and Eddy, Wastewater Engineering: Treatment andReuse, 4th ed., McGraw Hill

4. J.D.Seader, Ernest J.Henley, Separation Process Principles,2nd ed., Wiley & Sons

5. http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/wasteWater/index.htm

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