In 2004, Clark Public Utilities built a Sequencing Batch Reactor to treat a peak day of 1.0 MGD. Expansion plans for the Site was a peak day of 2.0 MGD
Built the headworks for gravity flow into the plant and process basins
Not using membranes we didn’t need will extend the life and reduce the costs to replace the membranes
The flow goes through fine screens in the headworks, parshall flume and into a channel splitting the flow into two trains, anoxic, swing , aerated. Feed forward design into the MBR and overflows into the channel and the recycle gravity flows to anoxic basins. Permeate is disinfected in inline UV and flows either to a storage tank for the utility water or out to the river.
Solids are sent to an aerated storage tank, transferred to a day tank and then dewatered with a rotary fan press and dried in a batch dryer to meet class A EQ solids for use at Lewis River reforestation. Utility water is used for the press and dryer and recycled for treatment.
Probably the most unique part is the growth in the city quit at the same time we were doing the construction of the upgraded treatment plant and it has continued for a few years which has allowed a good comparison between the two treatment systems. The flows and loads have not varied significantly. The flow in 2011-2012 in inflated by the recycle from the press and dryer initially being sent through the screens and influent flow meter. It is now discharged into the recycle channel downstream of the meter.
Because we converted the SBR tanks to the process tanks we were limited in some flexibility. One problem is that we cannot isolate part of the process train without removing an MBR as well.
The basins with the existing program would run both tanks at lower flow rates and not run one at high.
Removing one tank and cleaning and storing the membranes seemed to be risky. It would reduce our ability to clean the operating MBR, not be available if we had a high flow event etc.
The big change was in the tons of solids produced in the SBR compared to the MBR and the quality of the effluent discharged from the facility. We have seen an 88% reduction in pounds of TSS discharged and 53 % reduction in pounds of BOD. We are almost always below the detection limit on both the BOD and TSS in the effluent discharged.
Operational costs were for items like polymer, lab supplies, cleaning chemicals
Maintenance costs were parts, UV bulbs, outside personnel, electricians, mechanics, etc
Utilities were water, natural gas, electricity
We did not include the labor expenses in the evaluation as no changes were made in staffing and we have not had a significant change in overtime. One of the benefits of the MBR is we have reduced the time operators spend in the lab and have been able to increase the preventative maintenance and do more repairs ourselves rather than sending things out for repair.
As you can see the Plant electrical use increased significantly. This was offset by the ability to reuse the effluent for the dewatering and drying units. This was a reduction in the purchase of over 7 MG per year of potable water. The use of natural gas dropped with the reduction in solids produced by the MBR. The SBR produced 0.9 lbs biosolids per pound BOD, while the MBR produces 0.6 lbs biosolids per pound of BOD
We had two operational modes. One was a fill and permeate or level controlled. The other was a program that looked at incoming flow and allowed for some fluctuation in the basins to be able to cut off high peak flows.
The flow based program, contained all sorts of correction factors which were difficult to fine tune. Both programs would allow the system to operate on one or both basins on a low flow setting ½ Q which prevented it from being energy efficient.
Ovivo sent an integrator out to work with us on optimizing the control system to do what we were doing manually. Russell Reed came out and worked with us on a program that would allow the system to work with only one MBR online, maintain the basin levels, shut off one or both basins, and allow the operator to adjust setpoints to optimize the permeate flows and air scour.
By using the volume of the tanks we had for equalization it would allows us to permeate at medium to high flows and then allow the basins to go itno intermittent mode until the basins filled back up.
We started with energy efficiency because of the increased electrical costs. We looked at each section of the plant to see what each was costing and how to reduce it. So the 2011-2012 was the baseline. The 2014 was after we had redone the programming in the basin. And the 2015 is the goal for this year.
Utilized the “Process Trends” to track where the most air was being used
Looked for ways to reduce the usage
Looked at electrical use of other equipment in the plant
Current minimum flow rate is 160 gpm
Next step is increase it to 180 gpm which will put the maximum flux at about 16.5 and the average should remain the same
The baseline for 2011- 2012 for the MBR section of the plant. In 2014 after optimization the electrical cost for the Air Scour dropped by 22 %. I am predicting the cost to drop by more than 30 % for the 2011-2012 baseline in 2015. This can be accomplished by increasing the minimum flow setpoint to 180 gpm. This will put the instantaneous flux rate at 16.5, but it will maintain the same average daily flux as the plant will permeate for fewer minutes.
An MBR plant can be optimized to be energy efficient. The cost to operate the City of La Center plant was 13.6% less than the SBR at the same flows and loads and after we went through an optimization project we increased the savings to 19% and expect to be able to realize additional savings.
With optimization the operational costs were reduced to be 19 % less overall compared to the SBR plant the City had been running for 4 years.
With further optimization the electrical costs could be reduced further with little or no impact on the MBR. Some additional treatment components added significant electrical costs, for example odor control, which is not directly related to the electrical cost of the MBR. Need to be able to look at apples to apples if possible when making comparisons.
