Alum Control Alternative Assessment

Transcription

1 Alum Alternative Assessment Tobin Synatschk, PE, Water Practice Leader Jones and Carter, Houston, Texas In 1998 the Environmental Protection Agency (EPA) mandated that the country s states impose limitations on nutrients entering its surface water resources. The primary nutrients targeted are nitrogen and phosphorus for their ability to severely impact the quality of the nation s surface waters. In extreme quantities, these nutrients can cause eutrophication which is the rapid growth of algae, commonly referred to as algae blooms, and hypoxia or areas of rapid phytoplankton growth. Inland blooms are very unsightly, can kill aquatic life by reducing the dissolved oxygen concentrations and impart taste and odor problems for drinking water plants. In recent years hypoxia has received media attention as the limits of the dead zone in the Gulf of Mexico have been mapped. In accordance with the EPA s mandate, the Texas Commission on Environmental Quality (TCEQ) began the laborious process of quantifying the problem through stream testing, identification of sources, a review of technological capabilities for nutrient removal and establishing a priority for nutrient reductions. The TCEQ s State Implementation Plan calls for the reduction of nutrients and as such the TCEQ has begun writing discharge permits with technology based limits. Central and South Texas have seen the majority of these new limits because of their clear-running streams and drinking water impoundments. For New Braunfels Utilities (NBU) this would impact its North and South Kuehler wastewater treatment plants since they discharge into the Guadalupe River upstream of Lake McQueeny. In 2011 NBU started the design of its phosphorus removal system which uses chemical precipitation to remove the phosphorus to a concentration less than 3 mg/l. The chemical selected by NBU for its system was alum for its effectiveness, reduced handling hazards, its relatively high phosphorus discharge limit and familiarity of use by its staff. With the chemical selected, NBU performed a pilot study to evaluate the benefits of two alum control strategies: flow-paced control and residual control. The pilot consisted of the installation and operation of a chemometric spectrometer orthophosphate analyzer that continuously monitored the orthophosphate concentration at the tail end of the aeration basins. The primary objectives of the pilot were to determine: 1. The orthophosphate concentrations at the end of the biological process. 2. Does the orthophosphate concentration trend with the effluent flow rate making flow pacing possible? 3. Is it possible to reduce the amount of applied alum with an analyzer and if so, what are the monetary savings? 4. Would flow pacing alone provide the degree of treatment necessary to comply with the permit limits?

2 Thur 12:00am Fri 12:00am Fri 7:00pm For a three week period the analyzer was used to monitor the orthophosphate levels in the plant. The first two weeks it was installed at the North Kuehler plant and the third week at the South Kuehler plant. As shown in the graph below the Ortho P levels and flow through the plant were closely trending in a very consistent manner. On June 28 th the region experienced a rain event which caused the flow through the plant to increase. Orthophosphate levels dropped considerably and whipped from high to low over the next 3-4 days even though the flow returned to normal. Despite this swing in concentrations the Ortho P levels still showed a strong correlation with plant flow. The peaks and valleys in Ortho P corresponded with the peaks and valleys in the flow with a delay of several hours. The potential causes of this will be discussed later North Kuehler WWTP Orthophosphate W/ Flow to Rain Event 4 Ortho P Flow The South Kuehler plant receives influent that is separate from the North plant and comes from a different area of the City. The data from the week of testing is shown below.

3 MON 12:00am TUE 12:00am WED 12:00am THUR 12:00am South Kuehler WWTP Orthophosphate W/ Flow to Ortho P Flow /8/2011 7/8/2011 7/9/2011 7/9/2011 7/10/2011 7/10/2011 7/11/2011 7/11/2011 7/12/2011 7/13/2011 7/14/2011 7/14/2011 7/15/2011 As with the North plant, the South plant shows a strong trend with flowrate. It also shows smooth Ortho P concentrations for the first two days much like the North plant. The switch from smooth concentrations to highly variable concentrations was not marked by a change in flow event this time and could be due to instrument scaling or most likely process changes by the operator. To determine the effectiveness of one control method over the other, we calculated alum dosages that would result from flow-pacing and residual orthophosphate control. The flow-pacing values on the graph below were determined by calculating the stoichiometric alum dosage required to remove orthophosphates to a concentration of 2 mg/l. Since during actual operation in the flow-paced mode the concentration of orthophosphate is not known, an operational adjustment factor is added which allows the applied dosage to be increased or decreased from the theoretical value. This is similar to the disinfection system flow pacing where slightly more chlorine is added than is absolutely necessary to meet permit. Once dosage values were calculated with a single, constant adjustment factor, the results were graphed over the actual theoretical values. The adjustment factor was then reduced to bring the applied dosage as close to the theoretical dosage without dipping below the theoretical value to ensure continuous permit compliance. This generated the green line on this graph.

4 Thur 12:00am Fri 12:00am Fri 7:00pm North Kuehler WWTP Alum W/ Flow to Alum Total Cost: Flow Paced : $ Analyzer led: $ Residual Period of over dosing alum Time of excursi on Flow-Paced 0.00 To calculate the dosage for the residual control the stoichiometric dosage was used without an adjustment factor since the concentration of orthophosphate is known at all times. Since the residual control dosage is exactly equal to the theoretical dosage there are only two series of lines on this graph. As shown on the graph there are periods where the two dosing strategies yield different dosages. This is due to the orthophosphate concentrations that do not track directly with the changes in flow. In the areas where the flow paced line is higher than the purple line, alum would feed at a higher rate than if controlled by an analyzer. This represents alum that is overdosed. Until June 28 th, flow pacing would yield alum consumption rates that waste very little alum. Beyond this date, starting on the 29 th, the orthophosphate concentrations were significantly suppressed. The difference between flow pacing and residual control became significant and alum consumption for flow pacing greatly exceeded the residual control consumption. On July 3 rd, the orthophosphate concentration spiked higher than normal and the theoretical dosage exceeds the flow paced dosage. This is a time when permit excursions might occur if flow pacing were used since less alum than necessary is dosed. At the South plant, shown below, similar results were found. The majority of the days during this analysis, the analyzer would have resulted in less alum consumption than a flow-paced system.

5 MON 12:00am TUE 12:00am WED 12:00am THUR 12:00am Alum Total Cost: Flow Paced : $ Analyzer led: $ South Kuehler WWTP Alum / Flow to Residual Period of over dosing alum /8/2011 7/8/2011 7/9/2011 7/9/2011 7/10/2011 7/10/2011 7/11/2011 7/11/2011 7/12/2011 7/13/2011 7/14/2011 7/14/2011 7/15/2011 If the cost of alum is considered ($0.76/lb or $0.83/gallon) the savings generated by the analyzer can be calculated. At the North plant, the difference in alum cost was $331 for an approximate annualized difference of $9,293 per year. Operation of the analyzer requires periodic maintenance, consumes reagents and requires the use of an air conditioned building which reduce this cost savings. Based on estimates from the manufacturer, and HVAC electrical consumption for seven months out of the year, these expenses are approximately $2,430 per year. The net savings to NBU would approach $6,863 dollars per year. With a capital cost of $62,000, this yields a simple payback of 9 years. Using the same methodology at the South Plant, the alum savings were $9,855 per year with a net total savings of $7,425 per year. The payback for the South plant was slightly less at 8.3 years. Other side benefits exist that were not included in this analysis but should be considered as well: 1. Since alum addition precipitates phosphorus, there is a significant increase in the sludge production of the plant. Reducing alum consumption will decrease sludge disposal costs and will decrease the payback period. 2. Alum consumes alkalinity. Reducing the alum dosage will help prevent nitrification inhibition and make ph control after disinfection easier. 3. A portion of the excess alum may remain in the system and will return to the head of the aeration basin via the return sludge. This alum may be available for continued phosphorus removal if it is not consumed neutralizing the organics in an enhanced coagulation mode. 4. An intangible peace of mind that comes with knowing that swings of influent phosphorus concentrations will automatically result in swings of dosage. In summary, the pilot test proved that the phosphorus concentrations were approximately 3 mg/l post aeration, generally trended with the effluent flow rate, and that an analyzer could pay for itself in a relatively short amount of time. Flow pacing alone at this plant has proved satisfactory for permit compliance, particularly when manual adjustments are made during periods of high flows. The generous effluent limit helps in this regard, and tighter limits in the future may necessitate analyzer addition at this facility.