Low-Power RTUs Enable Wireless Control

Jan. 16, 2017
System integrator RTUdirect replaced long runs of troublesome wiring to remote water treatment drip fields with wireless radios communicating from a base station to remote RTUs powered by solar panels.

Harvest-Monrovia Water & Sewer Authority needed a solution to control a high-maintenance wastewater treatment sand filter system and associated drip fields installed in a subdivision. Drip fields are a secondary water treatment process using underground piping to disperse water into soil.

This particular installation has two drip fields with two zones. Each drip field is 2,000-3,000 feet from the sand filter control panel (Figure 1). There was no way to restore damaged wires due to the housing density in the subdivision. A solution was needed quickly because operators were manually opening and closing drip field valves on a daily basis.

Harvest-Monrovia consulted system integrator RTUdirect for this project based on a successful retrofit the company had completed on its SCADA system using AutomationDirect programmable logic controllers (PLCs).

RTUdirect provides automation systems, products and services to a wide range of clients using its extensive experience in SCADA, water and wastewater applications and tank inventory monitoring.

RTUdirect designed and installed drip field remote terminal units (RTUs) operated via remote control using an AutomationDirect DL06 master PLC and two AutomationDirect DL05 slave PLCs. A unique feature of the drip field RTU is its low power use, with each slave PLC turned on for only short periods of time during the day.

RTUdirect reviewed the site, evaluated several solutions and settled on AutomationDirect hardware. The hardware was proven to be reliable in prior installations, and it included the Modbus communication protocol, both key requirements for the project.

Water & Sewer Authority origins
The Harvest-Monrovia Water and Fire Protection Authority was formed in 1965 to operate and maintain a water system for the supply of domestic water in the Harvest-Monrovia area of Madison County, Ala. The board made the decision for the authority to start providing sewer service to certain locations in the Harvest-Monrovia service area in 2002.

The authority started with 700 customers and now supports about 15,000 water meters and a population of about 42,000. The system started with two wells capable of pumping 800 gallons of water per minute, and now treats 12 million gallons of water per day with four state-of-the-art water treatment plants.

The system draws groundwater from the Tuscumbia-Fort Payne aquifer system using seven wells. A 10 million gallon per day plant treats water from four of the wells. A membrane plant treats 3 million gallons of water from two other well sites, with water from one well treated directly at the well site.

The authority also operates two state-of-the-art membrane wastewater plants, and two packaged wastewater plants with a capacity of about 1 million gallons per day. Sand filter and drip field installation are maintained in two dozen subdivisions.

The authority has established a Source Water Protection Plan approved by the Alabama Department of Environmental Management. There are many possible sources of contamination to the water source such as septic tanks and agriculture runoff, and the authority’s stated goal is to provide clean, safe drinking water for its customers. Meeting this objective requires a number of remediation methods, including the use of sand filters and drip fields to treat septic tank discharge.

Gray water treatment process
In this wastewater treatment process, every house in a subdivision has a septic tank, but without the traditional field lines or fingers. Instead, a subdivision sand filter accepts the gray water from all of the septic tanks in the subdivision for processing and disinfection. This treated water is then distributed to multiple drip fields. This arrangement allows houses to be built on smaller lots than houses with traditional field lines.

The gray water arrives at the sand filter and flows down through the filter, which contains a granular sand or fine gravel material, and is then pumped from an underdrain to the drip fields. This effluent pretreatment sand filter improves wastewater quality before it reaches the drip field. At the drip field, the pressurized effluent passes through valves that are opened by the control system for a predetermined amount of time, as opposed to using a float or on-demand type of control method.

The typical subsurface drip distribution system—the drip field—consists of loops of pressurized pipe with drip emitters spaced every 3-5 feet based on the type of soil. The drip emitters must release the effluent slowly and intermittently. This controlled drip release ensures the soil does not become too saturated and remains oxygenated.

A drip field can be used year-round and is less intrusive than above-ground systems because it is buried in the soil. The drip emitters are positioned 6-12 inches underground, which ensures the effluent is dispersed to the root zone of the vegetation and grass above. In this drip-field configuration, most of the effluent is eventually returned to the atmosphere through evaporation after permeating upwards through the soil.

Controlling the treatment process
The control system is installed at the subdivision sand filter control building, with radio links to two associated drip fields (Figure 2). Sand filters and drip fields are typically installed along with the other utilities and support infrastructure when a subdivision is developed. The drip fields are often thousands of feet from the sand filter control building, and low-voltage control wiring is usually buried to connect remote valves for the different sections of the drip field to the control building.

In this subdivision, the low-voltage control wire was buried for a distance of about 2,000-3,000 feet. As houses were built and backyard fences were installed, wires were cut and spliced so many times that they became unusable. Eventually, the wires were beyond repair. Since the subdivision was fully built-out with houses, replacing the wires was no longer practical.

The original control system consisted of a base RTU installed in the sand filter control building. Outputs from this RTU were connected, via buried wire, to the solenoids in the two remote drip fields. These solenoids activated valves to control the flow of water in two zones at each drip field. The new control system eliminated the need for buried wires.

The new base unit transmits control signals via radio to remote RTUs in the drip fields 2,000-3,000 feet away. The drip field valves are controlled via radio using the two new RTUs, each featuring a slave radio that powers up when it receives a wake-up signal from the master RTU. The remote RTUs energize or de-energize latching relays to control solenoid valves in the drip field.

These valves modulate the effluent water delivered by a pump at the sand filter control building. The master AutomationDirect DL06 PLC in the sand filter control panel sends Modbus commands and reads back a confirmation. Each drip field DL05 PLC then initiates power down, spending most of its time in the off state as communications from the sand filter control panel are infrequent.

Control system details
RTUdirect assembled three custom RTU control panels using the AutomationDirect DL06 and DL05 DirectLOGIC product lines of micro PLCs. The DL06-based RTU functioned as the base unit at the sand filter control building, and the two DL05-based RTUs were placed remotely in the drip fields to control the drip field valves (Figure 3). The low cost and extensive functionality of these units were critical in this application.

RTUdirect had already replaced RTUs using PLCs from other vendors with AutomationDirect PLCs, and a number of sand filter control panels had also been installed using AutomationDirect PLCs, so the selection of automation components for this project was a natural progression.

The remote locations do not have a power source available, so each remote RTU is powered by a 12 VDC marine battery. The PLC at each RTU uses a power-saving mode to allow it to operate from a low-cost 15 W solar panel, which charges a battery installed in the RTU.

Opening and closing the two valves in each drip field is infrequent, so the DL05 PLCs in the drip field are turned off between switching operations. The PLCs control latching relays for each zone valve, so the PLC can be powered down after the switching sequence as the relays latch in place. When control functions aren’t required, each RTU goes into a low-power mode.

Digi 900 MHz 1 W long-range spread spectrum radios provide the communication link from the sand filter control building RTU to each drip field RTU. The drip field RTUs have different Modbus addresses so they can be monitored and controlled separately. The radios are configured to power up the PLCs for a short time when communication is required, and then power down between communication cycles.

The radios were simple to configure for communications since the DL06 PLC supports Modbus read/write instructions. The MRX and MWX (read and write) instructions allow use of native Modbus addressing, eliminating the need for the typical octal-to-decimal conversion.

Each remote RTU was installed with an omni-directional antenna just 6 feet high, which was a challenge to the range of the low-power radios. There was also a communication delay associated with the sleep cycle of the radios that had to be accounted for during design. The programmable features of the DL05 Modbus communication commands allowed these limitations to be overcome.

This wireless solution saved tens of thousands of dollars compared with replacing the buried wire, and also was much less disruptive to residents. The system has been in operation now for almost 10 years and has saved innumerable trips to manually adjust valves. Reliability has been excellent, and no control system issues have been reported.

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