Advanced control systems
The Northeast Water Reclamation Facility (WRF) control configuration uses a dissolved oxygen (DO) monitor and adjustable timers, which control how long the blowers are on or off. The DO monitor determines how many blowers are needed during aeration to prevent overaerating.
The newer Shoal Creek WRF uses the SchreiberFlex process control system. In addition to a DO monitor, it includes parameter concentration monitors for ammonia, nitrates, and phosphates. It also has two dosing systems: one for the addition of a carbon source, another for metal salts. The programmable logic controller (PLC) serves as the “brains” of the system.
Fortunately, the older CSR plant does not need investment in massive, expensive structural additions to accommodate the projected lower total nitrogen (TN) permits. All that will be needed is an inexpensive control system upgrade.
At the Northeast WRF, when a timer times out, the system moves on to the next stage. During the aerobic stage, blowers are on and the basin is in the nitrification process; the blowers turn off at the end of the period, with the system continuously alternating between on and off.
While the blowers are on, aeration occurs. “Free” oxygen is supplied to the mixed liquor suspended solids by the blowers to sustain nitrification. Once the blowers have been turned off, free oxygen is quickly consumed by organisms in the basin, making it possible for denitrification to occur. Once the blowers are off long enough, anaerobic conditions occur, necessary in achieving biological phosphorus removal.
Using only time periods as duration controlling parameters poses some problems. First, these time limits are preset, best-guess estimates; the time frames will almost always be too long or too short. Secondly, no provisions exist for making adjustments in response to the actual conditions. Even if these best-guess estimates are fairly accurate, changes in the influent render the guess inadequate, necessitating re-adjustment of the parameters. Also, with timers alone, there is no way to determine if the biological objectives of each phase are being met, or if one phase is receiving more time or energy than needed.
These shortcomings can be categorized into two areas: questions regarding the effectiveness of the process, and about its efficiency. At the Northeast WRF, timer-based challenges have been handled through personnel's knowledge of the process, diligence, and attention to detail.