Proper Design of Sewage Treatment Plant

Domestic wastewater management system of any city consists of collection, treatment and disposal. Wastewater Treatment Plants serve diverse treatment applications, ranging from the ubiquitous Domestic Sewage Treatment Plants (STP) to highly specific treatment plants. The Basic requirements of any treatment facility, be it a domestic STP or a very specific ETP, to deliver satisfactory performance over the long term, remain the same.

Below is expert advice for Proper Design of Sewage Treatment Plant by Dr.Ananth, a leading expert on Sewage Treatment

Background

Wastewater Treatment Plants serve diverse treatment applications, ranging from the ubiquitous Domestic Sewage Treatment Plants ( STP) to highly specific treatment plants say, treating wastewater from a Palm oil processing facility, Pulp and Paper manufacturing facility,  Textiles processing industry, Petrochemical plant, Bulk Drugs manufacturing facility, all of which are commonly termed Trade Waste or Effluent Treatment Plants ( ETP).

The Basic requirements of any treatment facility, be it an STP or a very specific ETP, to deliver satisfactory performance over the long term, remain the same :

• Proper Design of Treatment Plant
• Proper Engineering of Treatment Plant
• Proper Operation and Maintenance of the Plant

All the three aspects above are interdependent, and can deliver performance only synergetically :  in many instances, the one cannot fully rectify or overcome lacunae and defects in the other two aspects of the plant.

Design Aspect of A Treatment Plant

Various aspects to be considered in the design of a treatment plant include :

• Characteristics of the wastewater in terms of quantity, quality
• Presence of toxicants/ biological inhibitors ( in case of a biological treatment plant)
• Wastewater discharge pattern ( Continuous/ intermittent/ batch dump/ peaking factors/ seasonal variations)
• Locational and environmental factors ( Altitude, Temperature variations, rainfall etc)
• Selection of appropriate Technology, treatment units and treatment Processes Etc

Engineering Aspect of A Treatment Plant

Engineering aspects of a plant are related to :

• Proper and optimal layout of plant
• Proper and optimal hydraulic levels and level differences of various units in the plant
• Proper selection of mechanical equipments such as capacity/ rating etc. to suit the Design and engineering of the plant
• Inter unit piping per good engineering practice
• Proper electrical system, cabling per good engineering pracitce
• Proper placement of equipments and units for ease of maintenance
• Proper illumination, ventilation etc., for operator comfort, health and hygiene, and incentive to remain at the plant for monitoring Etc

Operation and Maintenance Aspect of A Treatment Plant

Aspects to consider under this head include :

• Developing and maintaining a Standard Operating Procedure (SOP)
• Training of operators in the SOP
• Developing and maintaining a Preventive Mtce. Program and Checklist for mechanical equipments
• Trained mechanics and technicians to execute the PM program
• Developing and maintaining an operational Log
• Frequent Monitoring, Control and Validation of the above procedures and processes by qualified and experienced Field staff from H.O of the service provider / AMC contractor Etc

Typical Examples of Interdependence of Units in a Treatment Plant

Each and every single treatment unit and treatment process in the treatment plant must operate at its optimal, design efficiency for proper functioning of the Treatment plant.  A malfunction in any one unit can have a serious and adverse impact on one or more of other units in the treatment train.   A few examples will suffice for illustration purposes , all of them ultimately resulting in poor performance of the treatment plant, directly and otherwise.

a.    A malfunctioning/ defunct/ inaccessible Bar screen  ( a most innocuous looking unit) can damage the pumps in the Treatment plant, leading to down time and malfunctioning of the Treatment plant.   (Engineering defect)
b.    An undersized Equalisation tank will lead the operator to pump out water at higher than design rates, leading to overloading of the aeration tank, settling tank, leading to poor performance of the plant ( Design fault)
c.    Improper aeration/ mixing in the Equalisation tank will lead to solids settlement, leading to frequent choking of pumps/ septic conditions in the Eq. tank, again resulting in poor treatment performance ( Engg. Defect)
d.    Selection of submersible pump with very small internal passages for raw sewage lift duty will lead to repeated failure of pumps ( Engg. Defect)
e.    Capacity of Aeration tank not sufficient for complete treatment of the design quantity and quality of wastewater as per good engineering and process calculations ( Design Defect).
f.    Mixing in Aeration tank not proper due to insufficient number, size and specification of diffusers/ aerators ( Design Defect)
g.    Excessive difference in Water levels of Aeration tank and Settling tank leading to turbulence in settling tank ( Engineering Defect), having a cascading effect :

• i)    Solids carryover form the settling tank
• ii)    Overloading and choking of the filters that follow
• iii)    Inability to settle and capture microbes for recycle to aeration tank
• iv)    Inability to maintain desired level of MLSS in aeration tank resulting in a vicious cycle starting with i) all over again

h.    Undersized Filter press for sludge dewatering will result in inability to maintain proper MLSS  level in the aeration tank resulting the same cascading effects described in (g) above Etc. Etc.

Summary and Conclusion

• Proper Design, Engineering, Operation and Maintenance are absolutely imperative for successful and satisfactory performance of a treatment plant in the long run.
• Each and every unit must perform at its optimal and most efficient design level for overall satisfactory performance of the plant
• Each and every unit in a treatment plant directly or indirectly impacts performance of other units in a treatment plant
• No unit or process in a treatment plant may be considered as being of secondary importance due to synergy between units as described above.
• All Apartment Complexes with an STP will do well to check suitability of Treatment Technology, Adequacy of Design and good Engineering Practice in the execution of the Treatment Plant before accepting the STP from the Builder/ Developer.

Thanks to Dr. Ananth for contributing the article. Dr. Ananth S. Kodavasal is Director of Ecotech Engineering Consultancy Pvt. Ltd, Whitefield, Bangalore.

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