Sussex County Municipal Utilities Authority (NJ)— Composting Monitoring and Reporting
The Challenge: Monitoring, reporting, and controlling the temperature of a five-step sludge composting process.
The Solution: Developing a PC-based SCADA system networked with PLCs and written with LabVIEW.
Sewage sludge is dewatered and then composted in piles at the Sussex County Municipal Utilities Authority (SCMUA) waste treatment plant near Hamburg, NJ. Proper composting occurs within a two to three-week period when biological activity is carefully maintained by controlling the time/temperature profile of the compost pile. SCMUA must process the sludge as quickly and efficiently as possible with minimal odors and other effects on the environment. To optimize this process we developed TrendSafe, a LabVIEW-based supervisory control and data acquisition (SCADA) system, to interface with programmable logic controller (PLC)-controlled aeration. The five PLC- based control systems were developed by Howman Controls, Inc.
The sludge is mixed with wood chips and placed in piles 10 feet high inside five closed buildings. Each building contains six piles of the compost mix on a wood chip base. Six fans blow air up through the six piles. Five PLCs, one per building, control the air flow to each pile. Composting results from the activity of bacteria, which grow faster in a warm environment with an adequate oxygen supply.
Control System Overview
Each building has a PLC-based control system. The composting process can be monitored and controlled locally through an operator interface terminal (OIT) mounted on each control cabinet. The PLC-based control system in each building operates independently. If one building is disconnected from power for maintenance, the other buildings operate normally.
The system monitors pile temperatures via RTD probes. The fan operates depend- ing on the temperature and phase of the composting of that pile. The fan aerates and cools the compost pile. The bacteria in the material require a minimum level of oxygen for their metabolic processes; they generate significant amounts of heat as they digest the compost. If left uncontrolled, they might use up all the oxygen or the temperature might rise to a level that would kill the bacteria. To minimize the time needed for composting, the system maintains specific temperatures and a suf- ficient oxygen level.
Control of the Process Phases SCMUA has perfected a five-phase compost- ing process that results in efficient conver- sion of sludge into a salable compost mix.
- Pathogen Kill
- Cooldown (Some of which are illustrated by the tem- perature trend shown above).
The Startup and Flash phases use timed fan control to provide sufficient oxygen levels for the bacterial action to begin. The amount of air supplied is calculated to increase bacterial action (and pile temperature). We cycle the fans during the Startup phase until the temperature of the pile reaches 35° C. During the Flash phase, which begins at 35° C, we cycle the fans at different rates until the temperature reach- es 50 C. At that point, the Pathogen Kill phase begins.
Pathogen Kill, Compost, and Cooldown phases all use temperature setpoint control to hold the temperature at a particular setpoint. Pathogen Kill holds the pile at 57.5° C for a period of three days to destroy any bacteria or viruses that might be harmful to humans. After three days at 57.5° C, the Compost phase begins. Compost phase holds the pile at 55.5° C for an indefinite period. After a minimum number of days, the operator determines that the pile is ready for unloading and manually switches the pile from Compost to Cooldown. Cooldown Phase holds the pile at 20° C. This operation reduces steam and odor levels before unloading the com- post from the building. Once a pile is removed from a building, the pile is man- ually switched to Stop mode. In any building, each pile of compost is controlled separately. Piles can be in dif- ferent phases of the composting process. If a problem, such as power loss, occurs for a given building, it affects only the six piles in that building.
LabVIEW SCADA System
The LabVIEW-based TrendSafe computer system, connected via an RS-485 data link to each of the five buildings, displays the current status of each pile in each building either individually or collectively. A variety of trending charts is available; all process control functions, such as starting and stopping pile operations, are available via the supervisory computer. A wide variety of reports is available, many in tab-delim- ited text files for easy spreadsheet analysis.
- Pile Phase Control
- Alarm Activation/Bypass and Status
- Real-Time Data Base (RTDB) – services
- 1,650 tags, once per minute. Functions configurable for each tag include conver- sion from source units to engineering units, alarm scanning and message generation, filtering (lowpass, moving average, and median), and historical data storage.
- Tag Editor–Allows modifications to tag attributes
- DataHistorian–configures each tag, for automatic historical data storage at regular intervals.
- Alarming – Each time a new input value is passed to the RTDB, the alarm status is determined, using the last alarm status, the alarm parameters defined in the RTDB (including the alarm dead band), and the current input. All alarms and operator responses are logged to a text file.
- Information from a Single Building – displays 17 important variables.
- Key Information from All Buildings (See screens on the sidebar.)
- Historical Pile Temperature Trending – displays temperature trends for a select- ed pile. (See trend on previous page.)
- Automatic Tag Value Report Generator – steps the user through the process of specifying a required report.
The system has been operating successfully since September 1996. SCMUA regularly uses the software to prepare monthly reports to the New Jersey EPA. LabVIEW provided a very easy platform on which to build the SCADA system. We designed, installed, and tested the LabVIEW portion of the system over a two-month period.