SELECTION OF AUTOMATION EQUIPMENT
Abstract:In response to an ever-increasing demand for and utilization of automation in wastewater treatment, criteria were devised to aid in the selection of automation components. The criteria were based on over 10 years of experience implementing automated equipment at four wastewater treatment plants and 50 pump stations.
Initially the automation components were often placed in service with no prior experience or track record. The approach was essentially hit or miss. Reliability, compatibility and functionality were often lacking. Many systems were excessively complicated with high upkeep demands. Components often exceeded the wastewater staffs or private contractors abilities to maintain effective operability. The costs to maintain these systems were often not practical.
An example of failed automation would certainly have been the initial plant computer systems. The costs were in the hundreds of thousands of dollars, they took up large rooms, required significant air conditioning, had high up keep costs, and did little more than print out a pm schedule when working. These units seldom worked, however, due to memory limitations.
The original valve actuators, pH meters, D.O. meters and similar components essentially did not work, often due to poor equipment or due to the frequency needed for pm and cleaning. The initial systems were large, costly, and at best, only allowed for remote monitoring. Unfortunately, the initial attempts at automation were a failure.
Perseverance and continued automation improvements have changed the picture significantly. Cobb County has four treatment plants, each with successful automation packages. The facilities: RL Sutton − 40 MDG, South Cobb − 40 MGD, Noonday Creek − 12 MGD, and Northwest − 8 MGD are all very successful award winning facilities. The awards are certainly a result of a top-notch organization, excellent staffing, design and equipment. Since 1990, the four facilities have seven plant of the year awards, one national award and one regional award, combined. In addition over the last five years, the facilities have received 15 gold awards, representing zero violations throughout the year. It should be noted that the number of awards is limited due to the fact that the facilities compete against each other in two basic categories above and below 10 MGD.
A different organizational approach significantly improved the overall level and reliability if the facilities. Prior to 1990 the treatment facilities operated independently. During the 1990's, the laboratory, maintenance, electrical, and instrumentation services for the facilities were centralized.
One impact of the centralization has been to create an environment where the good and bad experiences at each facility have been brought to the table. This is certainly true for each facility's independent knowledge and experience with automation. Prior to each facility's expansion the plant staff as well as the instrumentation group and engineering collectively decide upon the preferred automation equipment. The successful implementation of automation has only grown with standardization based upon the proven track records of equipment selected upon the initially independent experience of each of the counties four facilities.
Every new expansion of a Cobb County facility now only increases and enhances the automation of the expanded facility. It follows that the facility with the latest expansion has, at least temporarily, the highest level of automation. The last expanded facility was the Northwest Cobb WRF. So at least for a few more months will have the highest level of automation of the four facilities. This level will actually fall below that of the other facilities, temporarily. The next Northwest expansion will soon be underway. These discussions will focus primarily on the Northwest facility from here on out, due to its current level of automation.
Northwest is an 8 MGD advanced wastewater facility. The plant has 4 EQ basins, activated sludge with chemical phosphorus removal, sand filters and UV disinfection. The staff of 12 is made up of one superintendent, one operations supervisor, one administrative specialist, 6 operators and 2 maintenance techs and one maintenance utility worker.
In addition, there are central maintenance crews, instrumentation and laboratory services. A vital part of the automation package is the ability to service the equipment when necessary. The operations staff handles routine component cleaning and calibration, also checks equipment such as DO meters, solids meters, and the UV system. Equipment troubleshooting and repair is accomplished primarily by the instrument group, which is supplemented by several instrumentation and electrical private contractors.
The facilities automation package includes a SCADA system utilizing 9 plc's and 5 pc's all networked together, with most plant functions being controllable from the computers. Monitoring and trending is readily available for most vital parameters including, influent and effluent flows, EQ levels, lime level and feed rate, primary DOB, aeration D.O., influent and effluent, aeration air flow by zone, blower output, mixed liquor suspended solids, return sludge suspended solids, waste flow, secondary DOB, UV intensity, and dosage, odor control pH and ORP, digester levels, chemical levels, and flow to the following effluent parameters, pH, ammonia, phosphorus, solids turbidity, transivity, and D.O. The trends are available in increments from 5 minutes to 31 days and include the high, low, and average for each trend chart.
The experience gained from building the automation package along with the automation package itself has allowed for in-house criteria to be established to aid in the selection of automation equipment. Several criteria are essential as follows:
What is the purpose of the equipment? Is there a track record on experience with the equipment? This is probably the most frustrating component of finding good equipment. The unknown, still with enough research, the market place often has several workable alternatives.
If good equipment is found, is there local support? Even the best equipment may perform unsatisfactorily with limited or inaccessible support. Parts support is critical; are the parts available, affordable, and what is the time frame it will take to get the parts?
Standardization can be valuable, are there other similar working components in the system? If so, utilizing a similar component may simplify the operation.
Compatibility is huge, with the rapid fire changes in hardware and software, many components will be at different levels. Everything on each side of a component must be compatible. It is essential to know what hardware and software is currently available and what changes are planned. It is then essential to ensure new component is fully compatible. It is best to hold the supplier accountable to giving everything necessary to tie in to your current and known planned future system (when available). Longevity is very important to insure the new component will not soon be obsolete or will work with your planned upgrades.
The costs to purchase, use, and maintain each system should be evaluated along with the ease of operation. Simple beats complex any day. Some may enjoy a challenge, but running a wastewater facility is hard enough without a challenging automation on monitoring components. If there is automation equipment out there and you can purchase it, but for what purpose. Each component is something else to maintain. Each should be of value, required, or something to enhance and make some improvements to the operation.
Will the component improve the quality of treatment; will it lower the overall cost? Maybe there is a safety benefit. The component may reduce the need for hands on operation allowing the operations staff to be reduced. There may be a labor reduction available. The component may allow for improved reporting and tracking and therefore operation by improved analysis.
Prior to purchasing automation equipment it is necessary that it is compatible with your facility and properly matched to the process it is monitoring. Just because a component is good and reliable doesn't mean it will work or be economically feasible to add to your system. Many times an automation component doesn't work because it was misapplied, sized incorrectly or could not be supported by the exiting facility.
Things to look at include, what is the processes typical range? Does it match the range of the equipment? What is equipments accuracy? Is the accuracy consistent with monitoring or permit needs? Will the equipment hold up in your all season harsh environment? Is it possible to safely and reliably mount the component out of the way and still be serviceable? Is the correct power available? The following is the selection criteria summary:
Document Type: Research Article
Publication date: 2004-01-01
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