By Stephen Kachur
In the mid-1990s, East Bay Municipal Utility District's (MUD) Orinda Water Treatment Facility faced a large fine and a cease and desist order from the Regional Water Quality Control Board for discharging inadequately dechlorinated water into a local creek. Upon analysis, the major causes were determined to be imprecise chemical dosing and inadequate chemical dispersion and reaction time prior to discharge.
The 175 MGD plant receives continuous flows at the south and north ends of the facility from two aqueducts. Excess incoming water that is not needed for distribution is immediately discharged into San Pablo Creek, a receiving stream feeding into San Pablo Reservoir. Because the flows to the plant have a chlorine residual, the excess water must be dechlorinated prior to discharge. The plant's settled filter backwash water must also be dechlorinated prior to discharge to the creek.
Removing chlorine residual in the discharge water always had been a difficult permit parameter for the plant to meet. Prior to discharge of excess aqueduct flows and backwash water, operators manually controlled the feed of gaseous sulfur dioxide for dechlorination based on the estimated volume of water discharged combined with periodic chlorine residual test results. However, with an NPDES permit level for chlorine residual of 0.0 mg/L (defined by the RWQCB as below detectable level according to Standard Methods), the plant's dechlorination system proved inadequate.
The excess aqueduct flows (ranging from zero to 80 MGD) were dechlorinated as they entered the plant's spillway, and residence time in the spillway was too short - about 10 seconds - providing little opportunity for process optimization.
The principal violations, however, stemmed from problems with dechlorinating settled filter backwash water. Sulfur dioxide was fed at the pumping station directing the filter backwash water to the plant's one million-gallon settling basin. Backwash water on the inlet side of the basin often has turbidity of more than 150 NTU. It was difficult, in such dirty water, to maintain good chemical mixing for adequate dechlorinating interaction. The high turbidity interfered with the chemical reaction, compounding the risk of chlorine residual being present when the settled washwater was finally discharged into the creek.
Engineered Solution
To address the problem of inadequate dechlorination, an engineered solution brought the adoption of three new procedures: 1) modulating feed rates based on real-time flow rate and continuous, online chlorine residual measurement; 2) moving the point of chemical injection to allow for process optimization; and 3) increasing and optimizing mixing energy to ensure thorough chemical induction and dispersion. The plant also switched from gaseous sulfur dioxide to sodium bisulfite.
Instead of dechlorinating only the excess flows, all aqueduct flows now are dechlorinated upon entering the north and south ends of the plant. This provides increased residence time for dechlorination prior to the discharge of excess flows to the spillway. Additionally, chemical is now dispersed in the filter backwash water at the discharge line just after the water leaves the settling basin.
Data from residual analyzers and flowmeters is transmitted to a plant-based SCADA system that monitors flow and residual chlorine data to ensure the facility is feeding chemical at an adequate level to meet its process criteria.
Chemical Induction & Dispersion
The old chemical dispersion system consisted of feeding gaseous sulfur dioxide over diffusers to flows exiting the plant's discharge points. The new dispersion system uses flowmeters, residual analyzers, and Water ChampRegistered mixers to modulate chemical feed. The high speed submersible mixers are one of the keys to its effectiveness. Provided by USFilter's Stranco Products, Bradley, Ill., the mixers disperse chemicals directly into the process stream at velocities up to 60 ft./sec. Three of these mixing units were installed at the Orinda plant to optimize dechlorination.
The chemical induction/mixing units operate on the simple principle of applying all available energy directly to the chemical being activated. The unit design includes a 15-hp submersible motor with a titanium vacuum body and airfoil design propeller. Chemical solution is injected into the body of the unit, then simultaneously mixed and axially dispersed into the channel by the airfoil propeller.
The two mixing units treating aqueduct flows at the Orinda Water Treatment Facility were installed to optimize mixing and dispersion of chemical. The two raw water pipelines entering the plant pass under creeks into the north and south ends of the plant's riser section. A mixer was installed in each of the two ends of the riser section to inject sodium bisulfite into the water streams as they flow into the plant's raw water channel.
Positioned in the center of the pipelines, the chemical induction mixers create a high velocity countercurrent mixing zone that extends across the entire flow cross section. As the flow passes through the mixing zone, a homogeneous mixture of sodium bisulfite is instantaneously formed.
To improve settled filter backwash water disinfection, sodium bisulfite is now injected immediately after the water is discharged from the settling pond. A third submersible mixing unit installed at the beginning of the discharge line ensures proper mixing. Since this discharge line is only 200 feet long, rapid mixing and thorough chemical dispersion is imperative.
Improved Performance
The new chemical induction technology adopted by the water facility, combined with online residual and flow measurement, has provided a reliable solution to the plant's problems with dechlorination. The plant significantly has improved chemical efficiency, greatly reducing the potential for permit violations. As a result of installing the new dechlorination system and demonstrating performance, the plant is in compliance with its discharge permit. The regional board lifted its cease and desist order.
About the Author:
Stephen Kachur, P.E., is Assistant Superintendent at East Bay Municipal Utility District.
