Reevaluation of the design of STP at TUDr. Akepati S. Reddy

Department of Biotechnology and Environmental Sciences

Eligibility conditions for the contractors to participate:

Since fluidized aerated bed bioreactor is not very specialized and technology intensive process and since the design of the STP is separately provided, from the eligibility criterion-1, the words ‘based on fluidized aerated bed bioreactor’ can be eliminated.

Since capacity does not really matter much, in the eligibility criterion-1, the words ‘similar capacity’ may be replaced by the ‘≥0.5 MLD capacity’ words.

Design of sewers:

The sewers were wrongly designed.

Self cleaning velocity of 0.6 m/sec. has been indicated as considered but in none of the sewers it has been ensured.

Hydraulic elements like d/D and v/V, for the sewers for the present peak flow and for the design peak flow have not been worked out and flow velocity through the sewers during present and/or design peak flows have not been found.

The requirements of minimum crown cover of >0.9 m has not been satisfied (the crown cover has been as low as 0.35 m).

Intermediate sewage pumping stations:

Capacities of the pumps provided (200 LPM and 750 LPM) are bound to cause sewage backing up in the sewers when flow is closer to peak flow.

Sewage load:

For hostel population the water supply is stated as 150 LPCD. But this value has not been used in the calculations of sewage load assessments.

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Raw sewage characteristics considered:

Organic nitrogen and phosphate concentrations in the sewage may be required but not included. Given ammonical nitrogen and coliform count values are on the lower side (4 mg/L of ammonical nitrogen against typical >20-25 mg/L and 1x105-1x107 MPN/100 ml against typical 1x108) and the coliform count. Nitrate nitrogen is generally not expected in the raw municipal sewage and thus may not be required.

Sewage treatment process selection process:

Process selection is very vague, not that rational and defective and not sufficiently supported by data/information. It is excessively biased towards FAB technology.

When the sewage treatment is supposed to be by preliminary, primary, secondary and tertiary treatment processes/operations, the selection process is limited just to the secondary treatment process.

Though as many as nine selection criteria were chosen, they were not used in the true spirit. Pre-selection and exclusion of technologies is not rational.

Sewage treatment scheme:

The treatment scheme includes certain redundant units. Pressure sand filter and the activated carbon filter are not required. Effluent from the secondary tube settler (after the FAB reactor) is stated to have <30 mg/L of BOD and <50 mg/L of TSS. Further, chlorination of the effluent, after the biological treatment, is supposed to achieve <1000/100 mL MPN. The treated final effluent is supposed to meet the quality of 30 mg/L of BOD, 50 mg/L of TSS and 1000/100 mL of MPN. Under these circumstances, pressure sand filter and activated carbon filter are not needed.

The scheme indicates inclusion of oil and grease trap but it has not been designed into.

The document indicates inclusion of a fish/polishing pond of 7 m x 10 m x 1.2 m dimensions into the treatment process but that pond is not seen in the design.

Filter feed pumps and level switches for regulating these pumps are indicated in the design but no sump is provided for the water to be filtered.

Inclusion of FAB reactor and elimination of primary treatment are making the treatment process very costly. Air requirement is 650 m3/hour while it can be kept <250 Nm3/hour.

FAB reactor because of the costly media and because of the higher diffused air requirements (0.067 m3/m3.min. against the typical values of 0.015 to 0.020 m3/m3/min. fro ASP), since sludge recycling is still not eliminated, will prove very costly. The desired 88% BOD removal efficiency can be achieved by alternate cheaper technologies.

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Tube settler may not be that right choice for the secondary clarification, specially when MLSS levels are higher, and when there is risk of the floating FAB reactor media flowing into the settling unit.

Since the sludge will be sufficiently stabilized and have good settling and dewatering properties, a separate sludge pit with provision for aeration mixing may not be required and the sludge can be directly conveyed to the sludge drying beds.

Disinfection of the clarified effluent from tube settler is not going to be effective at typical doses of chlorine because of the ammonical nitrogen (~15 mg/L or more) and the TSS (~50 mg/L) in the effluent. Higher doses of chlorine are neither affordable nor desirable.

Screen chamber:

The bar screen and the approach channel are wrongly designed.

Depth of flow and width of the approach channel were assumed (as 0.5 m and 1.5 m respectively), rather than designed, and the required 0.3 to 0.6 m/sec. horizontal flow velocity can not be maintained in the approach channel.

Though designed for peak flow, the peak factor is taken as 2.5 while it has been taken as 3 for the sewer design.

MS bars with epoxy paint may be prone to corrosion and will get damaged quick.

Raw sewage sump and pumps:

Effective volume of the sump is insufficient. If the sewage is pumped at average rate of 42 m3/hour, during 9-00 AM to 2-00 PM the sewage will be backing up in the sewerage system.

Instead of providing four pumps, two of 50 m3/hr and two of 30 m3/hr, providing three pumps of same capacity can prove economical.

Stilling chamber with fine bar screen:

The stilling chamber with fine bar screen is wrongly designed.

Depth of flow and width of the approach channel were assumed (as 0.5 m and 1.5 m respectively), rather than designed, and the required 0.3 to 0.6 m/sec. horizontal flow velocity can not be maintained in the approach channel.

MS bars with epoxy paint may be prone to corrosion and will get damaged quick.

Drop to compensate the headloss across the screen is not provided.

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Grit chamber:

Grit chamber is wrongly designed.

Length of the grit chamber is calculated as 1.16 m. The length of 6.0 m provided is too less for a grit channel. While the required horizontal flow velocity is 0.3 m/sec. actual velocity achieved is shown as 0.0193 m/sec.

Though terminal settling velocity of the grit to be removed is calculated, it is not used in the design of the grit chamber.

With surface loading rate of 112 m2/m2.day the grit chamber will work more like a primary clarifier (for which the surface loading during the peak flow is 80-120 m3/m2.day) and in addition to grit even the other (putrifiable) suspended solids will settle here.

FAB reactors:

85% of the standard oxygen transfer efficiency is taken as field capacity for diffused aeration system. With the membrane fouling and higher sewage temperature this factor can not be more than 0.5.

Secondary tube settler:

When hopper bottom with 45 angle is provided, sludge scrapping mechanism (secondary clarifier mechanism) is not required in the secondary tube settler.

Sludge handling facilities including the sludge drying beds:

Sludge quantity is underestimated at 65 kg/day and sludge consistency will be >1%. It will be above 100 kg/day even after deducting the TSS in the clarified effluent.

Provisions are not made for the aeration mixing of the sludge in the sludge pit (requiring 37.5 Nm3/hour of air).

144 m2 sludge drying bed area may be insufficient. Assuming 20 day sludge dewatering and drying cycle, sludge loading will come to 0.83 m while typical loading is 0.25 to 0.3 m.

Grain size of the filter media (gravel and coarse sand) is not specified.

Disinfection:

Chlorine dose of 2 mg/L is too less for achieving the MPN of <1000/100 mL.

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One hour retention time for the chlorine contact time is too much (typical is 15 to 20 minutes)

Is it not better to go for hazzle free brine solution based electro-chlorinators rather than going for bleaching powder.

Filter feed pumps:

With the 30 m head a maximum pressure of 3 kg/cm2 can not be ensured in both the pressure sand filter and the activated carbon filter which are connected in series.

Pressure sand filter:

With only one pressure sand filter in place what will be done with the chlorinated effluent during backwashing of the pressure sand filter.

Filter medium depth of the pressure sand filter is quite high (1400 mm).

Effective size, uniformity coefficient of the filter medium and filter backwash velocities are not specified.

The pump and the water to be used in the backwashing are not indicated.

Activated carbon filter:

With only one activated carbon filter in place what will be done with the chlorinated effluent during backwashing of the filter.

Information on the exhausting of the activated carbon filter and the needed frequency of replacing the exhausted activated carbon with fresh activated carbon are not specified.

Others

The design document does not include hydraulic design of the STP and heads of different pumps have been decided apparently without the hydraulic design.

There are quite a few inconsistencies, like the following, in the design document:

On page 83 of the design document coarse bubbling type aerators are indicated but the same are stated as fine bubble air diffusers on page 97.

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