Presented with thanks to Dr. Suda Bunduwongse and Mr. Preecha Kaewkla (Tyco Earth Tech Thailand), Mr. Rick Bitcon (Earth Tech Canada), Mr. Steve Kirkwood (Earth Tech UK) and Mr. Ken Doyle (CFD Solutions, UK).
When Earth Tech engineering teams based in Thailand, Canada and the UK, designed and built the 4th in a series of 7 large wastewater treatment plants for the Bangkok Metropolitan Administration area, FLOW-3D was used to optimize the design of critical components in the treatment stream. The BMA-4 treatment uses the Earth Tech proprietary CASS™ (Cyclic Activated Sludge System) SBR process and treats wastewater from sources in the northern districts of Bangkok encompassing an area of approximately 12.9 square miles.
As with most other large wastewater treatment plants serving the Bangkok metropolitan area, the site was surrounded by existing roads and buildings and split by a drainage canal. Since these site conditions were similar to those at the BMA-2 CASS™ plant, which was constructed on four stories, the same design concept was adopted for the BMA-4 plant. A vertical design concept such as this requires innovative designs, particularly with respect to Preliminary Treatment and the CASS™ feed. Elements of these systems and subsequent problems associated with the cascade outfalls were the primary focus of a CFD investigation using FLOW-3D.
BMA-4 Wastewater Treatment Facility
The BMA-4 Wastewater Treatment was designed with:
- an inlet pumping station,
- preliminary and secondary treatment works,
- sludge treatment and odor control facilities, and
- treated water discharge.
The Earth Tech design team identified the Pre-Treatment (fine screens, grit removal, FFT control and storm overflow) and Secondary Treatment CASS™ feed (pumping station, sumps and feed chamber) as the primary elements of the BMA-4 plant design that could be improved with CFD analysis. Subsequent problems with scour near the treated water discharge cascades provided additional challenges. Earth Tech and CFD Solutions worked to develop different FLOW-3Dmodels of these systems and to communicate the results of the studies to teams of engineers based in Thailand and Canada at an early stage in the plant’s design, enabling savings in design time and cost and highlighting design changes which could improve plant performance at the start of the design cycle.
Preliminary Treatment Analysis
FLOW-3D was used to analyze a series of elements in the Pre-Treatment stream at the BMA-4 plant, including Inlet Chamber & Bellmouth Inlets, Screen Chambers, Grit Removal Units and Storm Overflow bypass and control.
The inlet and screen chamber model at right (illustrated with flow velocities) provided a platform on which to simulate various configurations of fine-screen operation and a resulting estimate of the flow split between the two outlet channels, which feed the grit removal units.
Simulations indicated the potential for inefficient balance of the flow-split to the grit removal units suggesting an overload of one of the units, which would be likely to result in non-optimal grit capture prior to secondary treatment. Detailed CFD simulation of the originally-purposed grit units also led to the replacement of these with alternative units resulting in higher grit capture efficiency.
Simulations also addressed some components downstream of the grit units, namely the automatic penstock control of flow to full treatment and the resulting storm overflow arrangements. CFD models of these process elements incorporated features unique to FLOW-3D (moving obstacles) to represent the transient operation of the penstock sluice control.
Free-surface modeling of the storm overflow weirs and computation of the complex 3D flow conditions and backwater resulting from the compact nature of these elements provided valuable additional insights relating to water surface elevations.
Secondary Treatment CASS™ Feed
Flow balance of the treatment storm to each of the four CASS™ basins on each story of the plant was modeled in a variety of configurations using the model show at right. The CASS™ process is a cyclic-continuous SBR method for which the operational strategies require one basin to be in “decant” mode during the cycle. The FLOW-3D model of the CASS™ feed sump was used to assess the flow volume balance to each of the basins filling during this condition.
Operational experience at the plant indicated some potential scour problems in the canal into which the treated effluent was discharged. Cascades installed to provide passive re-aeration of the effluent were causing local erosion problems in the receiving waters and FLOW-3D models were used to investigate various energy dissipating devices, which provided a successful remediation to the problem.
FLOW-3D was an effective tool for solving challenging engineering design problems at the BMA-4 Wastewater Treatment Facility. When this article was written, the facility was in phase-one operation, treating 59.4 million gallons per day.