Sugar Industry Effluent Treatment Plant. Evaluation and Recommendations

Evaluation of An Existing Effluent Treatment Plant With Recommendations For Revised Design In A 12000 TCD Sugar Plant

By Dr. Yashpal Singh

THE EXISTING SYSTEM

The following unit operations have been provided-

1. Oil and Grease Traps, 2 Nos.
   • Length, 3.5m each
   • Breadth, 1.5m each
   • Depth, 1.5 meter (SWD) each
2. Equalization Tank –Capacity 166 M³
3. Primary Clarifier
   • Shape-Circular
   • Diameter -12 meters
   • Depth -4.55 meters (SWD=30, FB=0.4, Sludge Hopper= 1.15 meter)
4. Aeration tank
   • Shape rectangular, in three compartments of9 meters each with a Max length of 51 meters and a Minimum Length of 45 meters
   • Total Area = 1285 sq meters
   • Depth= 3.5 meters SWD
5. Secondary Clarifier
   • Shape Circular
   • Diameter = 18 meters
   • Depth = 4.8 meters (3m SWD, FB 0.4m, Sludge Hopper 1.4 meter)
6. Sludge pits for primary and secondary sludges
7. Filtrate Sump – To collect filtrate from the Belt press.
   • Length = 6 meter
   • Width = 3 meters
   • Depth = 2.3 meters (2-meter SWD)
8. Influent pit for intermediate storage of filtrate before going to primary clarifier.
9. Belt Press
10. Sludge drying area
11. Dual Media Sand and Carbon Filters

Treated Effluents are proposed to be used for irrigation and when not in use for irrigation, to be disposed in to inland surface waters.

The licensed capacity is 14000 TCD but the actual capacity averages 12000 TCD. The evaluation is being done on 12000 TCD.

CRITERIA FOR THE EVALUATION

1. The following criteria for flow and influent quality have been assumed in the evaluation of the dimension provided in the existing ETP.
   • Crushing Capacity  12000 TCD
   • Permissible Effluent Flow 200 liters/TCD
   • Total Flow at 12000 TCD 2400 m³/day
   • Designed Flow rate for evaluation 2700 m³/day
   • Equalized Effluents 5 m³/hour Say 115 m³/hour
   • Temperature of Equalized Effluents 30 to 40⁰C
   • pH 5 to 6
   • TDS 1000 mg/L
   • BOD 1000 mg/L
   • Oil and Grease 30 mg/L

2. The Effluent treatment plant is designed to achieve disposal for irrigation and in the case of non-availability for irrigation, into inland surface waters. The plant is evaluated to achieve the following characteristics as per the revised standards prescribed for Sugar Units on 14-01-2016 by the MoEF and CC, Government of India.

Table

3. The current flow chart provides the following flow of effluents through the unit operations.
   • All untreated wastes will flow into the oil and grease trap.
   • From the oil and grease trap they would flow into the equalization tank.
   • Equalized effluents will flow to an influent pit which will also receive effluents form the belt press through a filtrate’s sump.
   • The effluents from the influent sump will be transferred to the primary clarifier.
   • Overflow from the primary clarifier shall go to the aeration tanks and the sludge through the sludge pit to the belt press. Solids will be taken for drying to sludge drying areas while the filtrates would be sent for treatment along with other effluents through the influent pit as shown in para d.
   • Aerated effluents would go to the secondary settling tank from where the overflow is sent to the dual media filter and finally to discharge. Sludge is recycled as required and the excess sludge is dewatered in taken for treatment as in d. above and the sludge sent for drying.

TREATMENT CAPACITIES AND EVALUATION

1. Oil and Grease Traps.

Two oil and grease traps operating in series and with the following dimensions have been provided

The design criteria for a trap prescribe that

• The length of the trap should be approximately twice its depth.
• Residence in the trap should be approximately 5 to 20 minutes. Preferably 10 minutes

• The surface area should be 1.5 times to 2 times the depth.

• The depth of the trap should not exceed 1.8 meters.
• Traps should be shallow to allow quick rise of oil and grease to the surface. Increased residence times do not result in increased efficiencies.

Evaluating against these criteria.

• The length to depth ratio is about 2.3 which is satisfactory.
• The surface area to depth ratio is 3.5 which is higher than the prescribed criteria.
• The residence time in each chamber is about 4 minutes and the total residence time in both is about 8 minutes.

Modifications in the existing oil and grease trap were examined to see how far they can be made to conform to the prescribed criteria. Examining for a situation where depth could be increased to 1.8 meters with the same area.

It is expected that by increasing the depth to 1.8 meters, the trap could be brought more closer to the design criteria. It is recommended that the Depth of the Trap be increased to 1.8 meters.

2. EQUALIZATION TANK

An equalization tank of 166 m³ capacity has been provided. No other dimensions have been given. The hydraulic retention time at an equalized flow of 115m3/hour is about 1 hour and 45 minutes. This could be an important reason behind the malfunctioning of the existing unit operation considering shock waves arising out of pump leakages, over flows, soda and acid wash waste, washing of mills, floor washing, molasses spillages, other day clearing wastes which may be periodic and create short circuiting in the system if not properly equalized. While ideally the equalization capacity can be a arrived at by an analysis of cumulative flows, yet in our case an assumption of 12 hours of equalization capacity would appear reasonably adequate.

 It is recommended that an equalization tank of a capacity of at least 1400 cubic meters and a mixing arrangement through diffused bubble aeration supplying at least 3000 m³/hour of air (at the rate of twice the volume of equalization basin or 3.0m3/m2 of floor area whichever is higher) may be provided to take care of shock loads and provide proper equalization. It is also recommended that separate storage capacities be created to store leakages and washings in small tanks near the points of generation and then transfer them in a controlled manner to the equalization tank. This would include pump leakages, overflows and spillages, soda and acid wash wastes, washing of mills, sulphur and lime house wastes, floor washings and molasses spillages.

3. INFLUENT PITS.

Influents storage pits have been provided to receive filtrate from the filtrate collection tanks and equalized effluents before they flow to the primary clarifier.

It is recommended that instead of the equalized effluents flowing into the influent sump, it could be the other way around and the Belt press filtrate as collected in the influent sump should be taken to the equalization tank in order to dampen its effects. It is expected to be a high concentration waste.

4. NEUTRALISATION PITS

Adequate neutralisation puts have to be provided.

5. PRIMARY SETTLING TANK

A primary circular settling tank with the following design parameters has been provided and evaluated for 2700 m³/day designed capacity.

The design criteria for primary settling tank prescribe a retention time of 2 to 6 hour for plain settling along with a surface overflow rate of 12 to 20 M³/M²/day and a depth of about 3 meters.

Evaluating against these criteria the existing tank satisfies the criteria for depth (3 meters SWD). With a surface overflow rate of 20 m3/m 2/day it also satisfies the criteria as above. The retention time is about 4 hours (3.6 hour) which also satisfies the prescribed criteria. The dimensions of the Primary settling tank, therefore, satisfy the design criteria but care should be taken that –

• Influents are distributed equally and at uniform velocities
• Flow enters the centre and is baffled to flow radially towards the perimeter.

• Weirs and Launders are levelled properly.

• Solids are not carried away from decanted waters, no mushroom cloud of solids is formed, there is no turbulence in clarifier and there is no flow of solids in the upper layers.

The dimensions of the existing primary clarifier appear to be appropriate to the proposed effluents discharge.

6. THE AERATION BASIN FOR ACTIVATED SLUDGE

The aeration tank, Carrousel like, appears to be designed to work as a plug flow activated sludge process. The systems are designed to facilitate flow in a serpentine manner with the influent and sludge being fed from one end and the Mixed liquor leaving from the other end to the secondary clarifier. In such a system the oxygen requirement is higher at the inlet, decreasing with reactor length, being minimal at and exit.

The following design criteria are prescribed for these types of aeration basins.

                                                                                   An Aeration basin with an area of 1285 sq. meters and a depth of 3.5 metres SWD has been provided. The volume available is 4497.5 say 4500 cubic meters. An evaluation was made on these capacities as follows.

• BOD load per day =2700 kg
• BOD removed in equalization and primary settling@30% conservative estimates = 810 kg
• BOD in Kg/day to aeration =1900 kg/day
• FM Ratio =0.4
• Biomass required is BOD/FM= 1900/0.4= 4750 Kg/day
• MLSS required = 3000 mg/L or 3kg/M³
• Volume required = Total Biomass/MLSS = 4750/3= 1583.3 say 1600m³
• Calculated Hydraulic Retention time       =V/Q=1600/2200= 14 hours
• Volumetric loading rate is Total BOD/Total volume =1900/1600=1.18 say 1.2 kg/BOD per m³ per day of aeration basin at a basin volume of 1600 m³

The actual retention time, as per the tank volume, is almost 40 hours which is excessively high and may impact the recurring costs and the oxygen requirements. The volumetric loading rate on the actual volume of the aeration basin has been estimated as 0.42 kg BOD per m³/day (BOD/Volume = 1900kg.day^-1/4500 m³) which is below the required concentration. The long retention time also appears unnecessary and the existing aeration basin appears to be oversized for the effluents.

An evaluation was also attempted to examine the conformance to design criteria if the volume is reduced and only 2250 m³ of the existing volume utilised.

• Effective volume = 2250 m³
• Flows = 2700 m³/day

• Retention Time =V/Q= 2250/2700= 20 hours

• Volumetric landing rate = BOD/V 1900 kg/day/2250m³=0.85

This satisfies the design criteria. Further at a MLSS concentration of 3000 mg/L or 3 kg/kl., the tank will have a biomass of 6750 kg (volume of basin x biomass concentration). At a desired FM ratio of 0.4 it can treat upto 2700 kg/day of BOD (Total Biomass x F.M.Ratio)  which is the initial BOD load assuming there is a reduced efficiency of BOD reduction in the primary clarifier.

The Air requirement is the maximum of air requirements for BOD removal @ 60 m³/hour air per kg. BOD, air requirements for mixing @1.2m³/hour per m³ of tank volume and 2m³/hour/m2 of tank floor area. This works out as.

• On BOD considerations = 60m³/hourX80kg/hour=4800m³/hour
• On mixing criteria = 1.2X2250=2700 m³/hour
• On tank floor area = 2X750 =1500 m³/hour

The maximum being 4800 m³/hour, a provision of 5000m³/hour could be made through coarse bubble diffuser or the existing surface aerator.

In evaluating the existing surface aerators, the following have been assumed for 30⁰C

• Density of Air at 30⁰C=1.16kg/m³
• Oxygen Transfer efficiency= 2.22 lb O₂/hp. hour or 1KG O₂/hp. hour

• 35 cubic meters of air to contain 1 kg of Oxygen

Based on this assumption the Oxygen requirements are 1150 Kg. and the Power Requirement 1150 H.P. per Hour. The existing tank provides for only 160 HP aerators which appear to be insufficient.

It is recommended that the aeration basin volume be reduced to 2250 m³/day and diffused bubble aerators capable of supplying at least 1150 Kg oxygen per hour be provided. The excess capacity of the aeration basin could be utilised for other purposes. The basin could also be redesigned as a rectangular complete mixed system. The current design of the baffles creates a situation for short circuiting of effluents from the top of the second chamber directly to the 3^rd chamber.

7. SECONDARY SETTLING TANK

A circular secondary settling tank of the following dimensions has been provided.

• Diameter = 18 meters
• Depth =  4.8 meters (3 meters SWD, F.B. 0.4m, Sludge Hopper 1.4 m)
• Effective Depth = 3 meters
• Surface Area   =             254.34 Sq meters
• Volume           =             763 cubic meters
• Length of Weir =             56.62 meters

The following design criteria have been assumed for the evaluation

• Depth = 2.5 to 3.5 meters
• Surface overflow rate = 12 to 18 m³/m²/day
• Solid loading Rate = below 3.0 Kg/m²/hour
• Weir Loading Rate = Less than 50 m³/m

Calculations

• Surface overflow rate = Flow(m³/day)/Area (m²)=2700/254=10.62 say 11 m³/m²/day
• Solids load in Kg/hour =112.5( Flow in m³/hr) X  0(MLSS kg/m³⁾=337.58 Kg/Hour
• SolidS Loading rate = Solid Load/Surface Area=337.5/254= 1.33
• Weir Loading rates in m³/m/day= Flow in m³/day/Weir length=2700/56=48.214 m³/m/day

Evaluation

The Secondary settling provided at the premises

• Provides a depth of 3 meters which satisfies the prescribed criteria of 2.5 to 3.5 meters.
• Allows a surface overflow rate of 11 m³/m²/day which is nearing the prescribed range of 12-20 m³/m²/day and adequate taking in view the slightly deeper sludge collecting hopper (1.4 meters).
• Is capable of maintaining a solids loading rate of 1.33 Kg/m²/hour which is within the prescribed limits of 3 kg/M²/hour.
• Allows a Weir loading rate of 48.214 m³/m²/day which is within the limit of 50 m³/m²/day.

The existing secondary settling tank is adequately sized to achieve the desired results.

8. DUAL MEDIA FILTER.

The complete details of the Dual Media Filter are not provided but the provisions could be evaluated against the following design criteria. This would assist in the removal of residual suspended solids and bacterial cells from activated sludge treatment and consequently the resultant COD removal of 70 to 75%, BOD of about 50%, SS removals of 60 to 90% and TDS removals of 45-50% are anticipated.

A filtration rate of about 200 l/m3/ minute could be taken as the design criteria with an area of 8 sq. meter and a dia. of 3.25 meters for the filter.  A portion of the spare capacity carved out of the existing aeration tank could be used to hold effluents from the secondary settling when not being transferred to the Dual Media filter during maintenance and backwash. Back wash effluents could be taken into the equalization tank for introduction to the treatment cycle. A volume of 240 m3 with a depth of 3.5 meters could be provided and this secondary treated effluent sent to the Dual Media Filter when in operation.

9. UTILIZING THE SPARE CAPACITIES OF THE AERATION BASIN.

As presented in para 6.0, a spare capacity of 2250 m³ could be carved out of the existing aeration basin and utilised elsewhere. A recommendation has also been made at para 2 for an equalization capacity of at least 1400 cubic meters along with storage sumps for day washings and pump leakages. It is therefore suggested that the following units could be provided from the spare capacity available in the aeration basin.

• Equalization Tank with a volume of 1400 m³ at an existing depth of 3.5 meters SWD. This would provide an equalization capacity of about 12 hours which appear sufficient for Sugar Mill effluents. It needs to be a completely mixed equalization basin with diffused aeration. The air requirements for mixing could be the higher of 2 times the volume of the equalization basin or 3.0 m³/m² of floor area. This equalization basin would also receive regulated volumes of stored day washings; dual media filter back wash and belt press filtrate. Aeration could also achieve a BOD reduction of 10% in the equalization tank.
• A storage capacity of at least 4 hours needs to be provided to store effluents from the secondary settling tank when not being sent to the Dual Media Filter during maintenance or back wash. This can also be carved out from this existing aeration basin. A volume of 240 m3 with a depth of 3.5 meters could be provided and this secondary treated effluent sent to the Dual Media Filter when in operation.
• After making a provision of an equalization tank and storage for secondary treated effluents as above, a spare capacity of 610 cubic meters is still available. This could be utilised as a polishing pond of 10 hours capacity and store effluents from the Dual Media Filter before final disposal into irrigation or to inland surface waters for reuse if possible.

10. OTHER SUGGESTIONS

Additional Storage capacities need to be created for storage of high temperature discharge per day washing effluents, leakages and other washings and spillages.

It is suggested that the existing equalization tank with a holding capacity of 160m³ be used as a storage for these effluents in a well mixed condition to prevent settling in this basin. Effluents from here could be transferred in a regulated manner to the equalization tank.

The details of the belt press are not available but it should be evaluated for its handling capacities.

Neutralisation tank of 7 to 10 minutes retention with proper mixing needs to be provided.

11. CONCLUSION

 A unit operation wise recommendation is being given below.

• Oil and grease traps – Depth of the existing traps be increased to 1.8 meters SWD
• Storage tank for washing/leakage etc. (Utilise existing equalization tank for storage, Keep it properly mixed)
• Existing influent tank could be continued to beused for additional storage of belt press filtrate.Instead of equalized effluents flowing into the influent sump, the filtrates from the influent sump should be mixed with the general effluents in a controlled manner before they are transferred to the equalization tank or at the inlet of the equalization tank.
• A neutralization tank of 7 to 10 minutes capacity with an adequate mixing arrangement should be provided after the oil and grease trap and before the effluents are sent for equalization.
• The existing equalization tank should be utilized for storage of highly polluting and high temperature waste streams before they are sent to the new equalization tank in a regulated way. Local collection tank at the points of generation may also be provided.
• Equalization tank – The existing equalization tank is inadequate and has been suggested to be used as a storage tank for intermittent highly polluting streams. A new equalization tank with a depth of 3.5 meters and a volume of about 1400 m³ should be created out of the existing aeration basin and maintained as a completely mixed system.
• The dimensions of the existing primary settling tank satisfy the criteria and the primary settling tank subject to being properly constructed appears adequate.
• While the existing aeration basin is oversized, the air supply provisions appear in adequate. It has been recommended that 2250m³ of aeration basin volume out of a total of 4500 m³ available be utilized for the activated sludge process with an oxygen provision of at least 1150 kg per hours.
• Secondary Settling Tank – The existing secondary settling Tank is adequately sized to achieve the designed results
• Dual Media Filter- A dual media filter with a filtration rate of 200 liters/m2/minute has to be provided and the existing evaluated against this criterion.
• A volume of 240m³with a depth of 3.5 meters for the storage of (04 hours capacity every day) effluents from the secondary clarifier when not going to the Dual media filter due to maintenance and back wash could be carved out of the existing aeration tank. and sent to the D.M.F. when it is operational.
• A polishing tank of 610 cubic to meters capacity could be made available to store treated effluents from the Dual Media Filter before discharge/reuse if possible.
• The belt process should be evaluated for efficiencies based on the primary and Secondary sludge produced.
• Keeping all storages in a well-mixed conditions is desirable.