Improving Compliance Through Real-Time Phosphorus Control

Wastewater from beverage, food or dairy processing plants usually goes one of two places: directly back into a natural waterway, or to a municipal wastewater treatment plant for further treatment. Permits and regulations vary between the two, and the upper limit for phosphorus depends on location as well.

Traditionally, regulatory agencies test for phosphorus by setting up a water sampler downstream from a processing plant and taking samples at set intervals; for example once per hour. Then this composite sample is tested once a day, and if it exceeds the permitted level of phosphorus, the agency levies a fine. In order to avoid fines, plant operators test their own effluent periodically. The more often they can draw samples, the more accurately they can measure phosphorus over time and dose control agents more precisely. However, the labor cost of manual sampling is multiplied as the number of samples increases, so most sites choose a testing interval and hope that it is frequent enough to detect changes. To compensate, the sites overdose with FeCl ₃ to provide a safety margin.

The result of this approach is that sites often use too much or too little chemical and that is when they get fined. If, for example, their allowable limit is 1.0 mg/L, a site may set their dosing levels to achieve 0.8 mg/L based on the average phosphorus content of their effluent, hoping this will be sufficient to control variations. The intent is to reduce risk and uncertainty, but this does not really improve controllability. This strategy uses 20% too much ferric chloride most of time while not controlling sudden phosphorus spikes. Spikes can occur for various reasons. A process changeover or increase in process speed increases water flow, discharging more phosphorus. Cleaning operations might use phosphate-containing detergents and high pressure, high temperature water that can suddenly send higher than average amounts of phosphorus downstream.

In short, plant operators are hit with costs two ways – paying too much for dosing chemicals while still being fined for excess phosphorus.

Increasing frequency of grab sampling improves the chances of detecting changes in phosphorus levels, but these grab samples only provide a snapshot in time of stream conditions. Dosing rates are based on composites of grab samples over a previous time period, so operators are dosing for past discharges, not the current one.

Fortunately, there is proven technology for automating realtime monitoring and dosing control that can give beverage, food and dairy plant operators the data and control they need to meet regulations without overspending on chemicals. A system of compatible, integrated sample analyzers and dosing controllers all managed from a central control unit takes the guesswork and human error out of phosphorus treatment.

A complete, integrated real-time control system starts with an automatic analyzers. The Phosphax sc Digital Phosphate Analyzer from Hach® can prepare and analyze a sample in under five minutes. Set in a ruggedized weatherproof housing it can be set right at the tank to provide continual, highly precise measurements of phosphorus levels with detection limits as low as 0.05 mg/L. It is designed to use minimal amounts of reagent. Multiple output options are available, making it easily compatible with existing systems.

The analyzer sends data to a central controller, the Hach SC1000 Multi-Parameter Universal Controller. This solid-state, modular controller can monitor up to eight sensors directly or be networked to monitor 32 sensors, each analyzing different parameters. A large color touchscreen lets operators observe system status quickly. Parameters can be adjusted easily when processes change.

The central controller receives data from the analyzer and sends commands to a Phosphorus Real-Time Controller (RTC-P) that manages the coagulant (typically FeCl ₃) dosage in real time. It signals the feed pump to discharge the appropriate amount into the effluent stream.

The Hach RTC-P also includes PROGNOSYS™ software for predictive diagnostics. This subsystem continually monitors the RTC-P system and delivers status alerts so operators can take proactive troubleshooting, maintenance and repair action.