Repair and Rehabilitation of Sewage Treatment Plants

Anil Kumar, Regional Manager, S.L .Goel, Deputy Project Manager, Milind Kulkarni, Repair & Rehabilitation Expert, and K.B. Tandon, Project Engineer, MWH India Pvt. Ltd., New Delhi

The repair and rehabilitation of civil structures of sewage treatment plants (20 & 40 MGD and 4 digesters of 12 MGD) at Keshopur in Delhi has been taken up with a basic view to ensure that by doing so it is possible to extend the life of the treatment plant infrastructure by at least 25 years. This coupled with a retrofit of the electrical, mechanical, instrumentation, piping and pumping works does help in making these plants operational once again at a much lower costs than would be required for construction of new plants of the same capacity.

There are a large number of such treatment plants in similar state in the country and which are not being used on account of being in a dilapidated condition. A proper survey, followed by estimation, specification and quantification of such works can be done. This further followed by tenders for repair and restoration work will pave the way for reutilization of such infrastructures and assist the cash strapped Municipal Corporations and Local Bodies to effectively utilize their resources in a better way and put in place treatment facilities at much lower and affordable costs.

Project Background

Yamuna Action Plan, Phase-II (YAPII) was formulated after the YAP-I phase to further address the problem of pollution of River Yamuna, particularly in the 22 km Delhi stretch and downstream up to Agra. The sewerage works proposed under YAP-II are in Delhi and Agra. The Delhi stretch of Yamuna is considered as the most polluted stretch, with Agra being the second most critical stretch.

The main objective of Yamuna Action Plan, Phase-II is further abatement of pollution of River Yamuna generally by way of increasing the capacity of sewerage system through rehabilitation of old sewers / pumping stations, construction of new sewers and increasing the sewage treatment capacity by rehabilitation of the existing sewage treatment plants and constructing a few new sewage treatment plants.

The cost of proposed YAP-II has been estimated at a sum of 6,370 million rupees, which shall be in the form of soft loan from the Japan International Cooperation Agency (JICA) to the Government of India.

National River Conservation Department (NRCD), Ministry of Environment & Forests, Govt. of India, the executing agency for the said loan, have allocated a portion of the proceeds of this loan to the project implementing agency that is Delhi Jal Board (DJB) for implementation of the said works in Delhi. DJB will utilise these funds for physical implementation of identified works which includes finalisation of planning, design and execution / rehabilitation of several trunk sewers and treatment plants.

Introduction

As a part of YAPII Scheme, the following sewage treatment plant works will form one such Separate package for a turnkey contract:

1. Design, construction, commissioning, operation & maintenance of (i) 54.6 MLD (12 MGD) capacity pumping station and (ii) one module of 54.6 MLD capacity sewage treatment plant on turnkey basis at Keshopur, New Delhi.
2. Rehabilitation of 2 existing modules of 91.2 MLD (20 MGD) and182.4 MLD (40 MGD) capacity sewage treatment plants at Keshopur, New Delhi

The two existing modules of 20 MGD & 40 MGD capacity sewage treatment plants at Keshopur were constructed in late 70’s and late 80’s respectively. The scope of work for the rehabilitation of these two plants has been assessed through both a visual inspection of the sewage treatment plants and a desk top study. The investigation undertaken reveals that most of the civil structures (all concrete) are to be repaired and rehabilitated and some parts to be demolished and rebuilt.

The scope of rehabilitation of civil work envisages the repair, restoration and retrofitting of all the units of 20 MGD and 40 MGD sewage treatment plants and pumping stations located at Keshopur. Beside this four sludge digesters and one gas holder pertaining to 12 MGD Plant are also envisaged to be covered under the rehabilitation work.

History of Sewage Treatment Plants

The 12 MGD, 20 MGD and the 40 MGD plants were constructed in years 1956, 1976, and 1988 respectively. The useful life of the infrastructure for the 12 MGD was over having completed almost 50 years of service and hence a new one was proposed with the other two being taken up for repair rehabilitation and retrofitting with a view to extend their useful life by at least another 25 years.

Deterioration of Structures, Causes And Determination

The primary cause for the deterioration and damage of the structures is exposure to harsh Environmental conditions of rain, sunshine, and the summers and winters which accelerate the process of cracking of structures. Penetration of the moisture into such cracks further leads to corrosion of the reinforcement. The reinforcement expands and spalling of concrete occurs.

Poor maintenance of such damaged structures ends up with them being unserviceable at some point of time in the designed service life of the structure.

Further to this, the mechanical rotating and other such equipments also need to have a high level of maintenance. If for some reason these equipment fail and not put back into action then the Civil structure becomes defunct and eventually bypassed and neglected through a period of time.

The presence of gases and the temperature of the effluents along with the hydraulic jumps etc. also contribute to the primary damage which snowballs into major damage on account of lack of proper maintenance budgets, schemes and systems.

Repair and Rehabilitation

The repair and revalidation works are being implemented by achieving the following milestones:

• Condition survey and assessment of type and extent of damage.
• Special destructive and non-destructive testing for critical areas by renowned test laboratories.
• Listing of types and extent of distress based on the above findings and test Reports.
• Finalization of philosophy of repair and restoration and identification of appropriate products & systems.
• Specifications, methodologies of repair and bill of quantities to be executed.
• Working drawings for all major repair and intervention works.
• Training of engineers, supervisors and sub-contractors.
• Testing of waterproofing and repair materials.
• Final supervision during execution of work.
• Review and documentation.

The condition assessment survey of all the existing structures was one of the most important aspects of the study. A special team comprising of experienced trained supervisors, experienced engineers, riggers and quantity surveyors was required to inspect the structures for internal and external damage.

Simple steel and PVC hammers, Schmidt hammers and concrete markers were used to test and identify areas of damage to concrete, spalling of concrete, cracking, exposure and corrosion of reinforcement steel.

Locating & Listing of various types of distress, recording the same and quantification of such distress was the main goal of the survey. This was very important as further down the line when the philosophy of repair was finalized and the costing for each of these repair mechanisms worked out, it helped in working out a fairly accurate budgetary estimate.

The preparation of the budgetary estimate is a very important exercise in such projects. An appropriate provision in the form of contingencies has to be made taking into account the overall status of the structures so that cost over runs do not occur during the execution of the project.

The philosophies of repair, retrofitting and restoration were worked out for various types of distress based on a wide range of specialized repair and restoration materials available. This involved a deep study and review of a number of products and systems of various manufacturing companies and case studies of such applications.

Another big challenge faced during the survey was the planning for emptying of the structures and necessary bye passes to ensure that the plant keeps running and parallaly the survey works continues. In some cases of the receiving sumps it was not possible to empty the structure and hence based on visual inspection repair philosophies were finalized.

The digesters and gas holders posed a different problem. Since these were not in operation for a long period the sludge had hardened and no assessment could be done. It was decided that the work of dismantling of the RCC domes, removal of the sludge and condition survey including independent testing was made a part of the tender. It was also envisaged to additionally work out cost of dismantling of the existing domes, and reconstruction of new digesters and gas holders if the Test reports were to indicate that the existing structures were non-serviceable.

Specifications, methodologies of repair and bill of quantities

Based on the repair and restoration philosophies, the specifications for the various repair mechanisms were finalized. The specifications for the products were necessary to ensure quality control. The methodologies of repair were worked out for various damage or distress types as it involved using a number of products in a well defined sequence and style.

On the basis of the bill of quantities already worked out the detailed specifications were worked out for each of the sub repair activities. This was to ensure that the consumption of each of the items used for repair could be quantified and paid on item rate basis during the actual execution and the work was not done on area basis. This mechanism helped in getting the proper work done by the contractor as they were assured of payments for each and every work activity on an actual consumption basis.

Working drawings and training of the engineers, supervisors and contractors

The working drawings for all the major repair philosophies were prepared and linked with the specifications and bill of quantities. Sampling was done for each of these items and concerned engineers and supervisors were trained during this procedure. It was ensured that authorized applicators for specific products and systems were entrusted with the works. A copy of working drawing for a typical work activity is given as Annexure-A.

Repair philosophies

Various philosophies and technologies were used to ensure the durability of the repairs and restoration. They have been basically for repair of concrete, protection of steel, special plasters, waterproof coatings and gas proofing of digesters and gas holders.

• Zinc rich coatings for prevention of corrosion of reinforcement steel.
• Special bonding epoxies for bonding of old to new concrete especially for overhead repairs.
• Single Component, dual shrinkage compensated pan fib reinforced repair mortars (M60 Grade).
• Single Component pourable repair micro concretes for building up severely damaged sections.
• Special Design Grade Concretes having polypropylene and nylon fib to prevent micro cracking.
• Special Curing compounds to ensure proper curing in windy conditions especially for Domes.
• SBS based HDPE with aluminum foil thermo fusable membranes for gas proofing of RCC domes.
• Special pre-packed readymade plasters containing polymers, polypropylene fibre sand and cement.
• High grade epoxy coatings for the underside of Domes and high risk corrosion areas.

Testing of products materials and structures

Since the major products and systems especially epoxy based materials are available to DIN and other European specifications testing of the same in various test laboratories was a difficult and a long drawn process. However, most of the acrylics, polymers, repair mortars and epoxies could be tested to Indian Standards.

Testing of the repaired and restored structures will also be a part of the process and most of them being water retaining structures, the hydro test as per IS:3370 shall be mandatory.

Conclusions

The total project cost of the repair and retro fitting of civil structure for 20 MGD and 40 MGD plants together with the testing and repair of the digester (18 nos.) and gas holders (2 nos.) has worked out to Rs.480 Million. The execution of the works is in progress and is expected to be completed by June 2011.

The repair and restoration of these sewage treatment plants was taken up with a basic view to ensure that by doing so it is possible to extend the life of the treatment plant infrastructure by at least 25 years. This coupled with a retrofit of the electrical, mechanical, instrumentation, piping and pumping works does help in making these plants operational once again at a much lower costs than would be required for construction of new plants of the same capacity.

There are a large number of such treatment plants in similar state in the country and which are not being used on account of being in a dilapidated condition. A proper survey, followed by estimation, specification and quantification of such works can be taken up and further followed by tenders for repair and restoration work will pave the way for reutilization of such infrastructures and assist the cash strapped Municipal Corporations, Local Bodies and Zilla Parishads to effectively utilize their resources and put in place treatment facilities at much lower and affordable costs. This will also save a lot of time when compared to new construction.

Photographs

A few photographs of the key works are included in this paper for study purposes. These highlight both the complexity and criticality of the repair and rehabilitation works that were taken up under this project.

Acknowledgements

The Authors acknowledge their thanks to the Mr. V.S. Thind, Chief Engineer, Mr. V.K. Gupta, Superintending Engineer and Mr. P.K. Jain, Executive Engineer, DJB, the Site Engineering Staff and Supervisors of both DJB and Contractor M/s Vatech Wabag Ltd., Chennai for the tremendous efforts made by them in execution of probably the largest sewage treatment plant repair and rehabilitation project in India.

Acknowledgement

This article has been reproduced from the proceeding of 'National Conference on Repair & Rehabilitation of Concrete Structures' organized by ICI western U.P Gaziabad, IA Sructural Engg, and Association of Structural Rehabilitation, with the kind permission of the organisers.