Bangkok's multi-storey sewage solution
Bangkok's Yannawa WwTW was officially opened last December, marking the end of a $180M project started in 1994. The restricted area available for the plant required an extremely space-efficient treatment process and some novel sewer routing. John Senior of Ove Arup & Partners International reports.
On first examination, the problems appeared to be insurmountable. A portion of the existing flows, which previously discharged into Bangkok's system of khlongs (drainage canals), needed to be intercepted and diverted through a new system to a WwTW, while surplus flows were to be overflowed after screening.
Traffic congestion prohibited working on major routes during daylight hours, and only land owned by the Bangkok Metropolitan Administration could be used for construction to avoid delays in land acquisition.
Records of metropolitan sewerage to be intercepted were incomplete, as were plans of other utility services. The whole area was subject to land settlement of 20mm/annum and differential settlement problems between piled and unpiled structures needed to be carefully addressed. High land values dictated that the available WwTW site was less than 3 ha of which 50% was within the tidal zone of the Chao Phraya river and occupied by informal elevated housing on timber piles.
The absence of an integrated collection system prohibited accurate composite sampling to assess daily/annual variance in influent, necessitating maximum flexibility in the design of the treatment plant to cater for widely fluctuating flows.
And the appearance of the WwTW had to blend in with the adjacent commercial/residential river frontage properties.
In July 1995, the Bangkok Metropolitan Administration completed its technical and financial evaluation of bids and awarded the Yannawa Wastewater Project to the Samsung Lotte CEC jv for a contracted sum of $180M.
After contract award the initial problems were further compounded by the construction of an elevated mass transit railway, which was subsequently built along some of the proposed sewerage routes. Plus the financial crisis of 1997 increased the costs associated with the purchase of imported plant.
Solutions were formulated to overcome all these problems and, after successful performance testing, the project was officially opened, on time and on budget, on 14 December 1999, by the Governor of the Bangkok Metropolitan Administration, Khun Bichit Rattakul.
Yannawa is the first major environmental project to become operational in Bangkok. Other wastewater projects have been significantly delayed by contractual, technical and land acquisition problems.
The design approach involved detailed investigation of the existing pipe network and ground conditions because over 60% of the contract value related to the provision of primary and secondary sewerage.
Restriction on daytime working necessitated avoidance of sewers along main roads and an early decision was made to route the sewerage below the multiplicity of khlongs which criss-cross the city. This in turn necessitated the extensive use of trenchless technology and over 90% of the 51km of sewers were ultimately built using microtunnelling (for smaller diameters) and pipe jacking for main runs, up to 2.25m diameter.
The soft Bangkok clay, to a depth of approximately 15m, permitted jacking up to 200m in each direction from the jacking pits with jacking forces varying from an average of 600 t up to a maximum 1400 t. Jacking and receiving pits were built using braced sheet piles (up to 12m deep) and concrete caissons for deeper depths.
Three 20m deep sewage lift stations, built with diaphragm walls, were incorporated within the collection system to avoid sewer depths exceeding 15m, which would have necessitated jacking in the stiff clays, severely restricting jacking distances.
Pump station wet wells were located below the khlong bed levels and incorporated a series of submersible pumps with a combined capacity of up to 11m3/sec/station. The pumps lift into an outlet launder and the flow gravitates downstream. The pump stations have a rated capacity of 5 DWF, but in the event of prolonged power failure, backup in the incoming sewer ultimately overflows a weir into the outlet launder and gravitates to the treatment plant.
Under these surcharged conditions approximately 3 DWF is passed through the system. This prevents unacceptable pollution spills in the adjacent watercourses. Pump removal and maintenance access are achieved via shafts located in the footpaths adjacent to the khlong.
Limitations on space, the need for a WwTW structure that blended into the surrounding environment, the elimination of environmental nuisance and the flexibility to receive and successfully treat a range of influent strength derived from a combined sewerage system necessitated careful selection of treatment process. Typical green field treatment options would not fit on to the site and would be environmentally unacceptable.
A decision was made at bid stage to adopt a multi-storey plant using sequencing batch reactor (SBR) technology.
This provided real advantages in terms of savings on land requirements, readily available enclosed areas for air extraction and associated odour control, compatibility with surrounding development, ease of construction associated with reuse of formwork and cost savings associated with piling in poor ground. It was recognised however that additional operational costs were inevitable as a result of pumping a proportion of the sewage to higher elevations for treatment. Net present value analyses indicated that this form of construction was extremely cost effective in comparison with conventional plants, particularly if both roofing (for odour control) and land value costs were incorporated into the analysis. Furthermore, it became even more cost effective if pumping costs to alternative, larger more remote sites became necessary.
The Phase I full treatment capacity is 300,000m3/d with a preliminary treatment (screenings and grit removal) capacity of 1Mm3/d. The design caters for an ultimate full treatment capacity of 1Mm3/d with a preliminary treatment capacity increasing to 2Mm3/d.
The small footprint site necessitated a compact inlet and storm pumping configuration using submersible pumps within each of the two pumping stations. This provided additional flexibility permitting each of the stations to be decommissioned for maintenance/expansion while continuously maintaining flows to full treatment.
The compact approach to plant layout design was complemented by the inlet works arrangement using dynamic separation to remove screenings and grit particles. The total inlet works footprint occupies 25% of the area normally occupied by conventional preliminary treatment design.
Secondary treatment uses Earth Tech's CASS process (Cyclic Activated Sludge System) which is a variant of SBR technology. The process uses single tank aeration systems, and requires neither primary sedimentation nor final settlement tanks - making it ideally suited to sites with space restrictions.
Process selection was based on the intrinsic flexibility to meet final effluent requirements ranging from coarse to high quality standards - including nutrient removal - while accommodating a wide variation in hydraulic and organic influent load. This has been proven at Yannawa where the actual influent is significantly weaker in organic load and higher in DO content than had been expected. The CASS process has consistently achieved the required effluent standard. Surplus activated sludge is dewatered and thickened in belt presses before being conditioned with lime and conveyed to storage bunkers prior to disposal on land.
The plant relies on simple control routines which are easily automated. The control system is based upon networked PLCs operating in conjunction with a SCADA system which displays and records plant operation in a central control room.
A high level of simple and reliable process instrumentation is provided to facilitate automatic control of the process plant. The local workforce has been trained in a short period of time and is now carrying out O&M activities.
The covered building readily permits containment of odours which are then conveyed via a ducted extraction system to a multi-stage wet chemical scrubber for treatment before discharge through a high level stack.