Sewage treatment plant uses radio

Sydney Water used radio technology when constructing a 300 mm diameter pipeline to transfer tertiary treated effluent pumped from its West Camden Sewage Treatment Plant (STP) to a 60 ML earth dam on the Elizabeth Macarthur Agricultural Institute property, 9km away.

The STP recycled water pumps are controlled by a programmable logic controller using cut-in/cut-out level signals installed at the dam. The GSM coverage in the area is very weak and there is no PSTN line at the dam site, so the only option left for linking the dam site instrumentation with STP plant control system was by radio.

Factors that needed to be considered for the link planning included:

• Choice of frequency;
• Transmitter output power;
• Receiver sensitivity and selectivity;
• Spread method (FHSS vs DSSS);
• RF noise level;
• Antenna type and gain. Antenna gain depended on the link budget, but it was realised that gain and beam width were going to affect the efficiency of the link;
• Transmission Line (coax) Quality.

The company conducted the site survey and obtained the geo coordinates for the STP and dam site. After feeding all the details into path loss software, it was found that a line-of-sight system was just not possible between STP and the dam site, because of tall trees attenuating the signa. As a result, the 2.4 GHz frequency option was ruled out and the obvious choice was to use frequency hopping spread spectrum radio, at 900 MHz.

A radio survey was then carried out in which a temporary pair of radios was installed to determine the reliability of the link. At this stage, the choice of antenna was considered as critical there being no line of sight.

The tests used two SRM 6210E radios, a 9 DBi omni antenna and a 12 Dbi Yagi.

At the STP, a 9 Dbi omni directional antenna was installed and at the dam site a 12 Dbi Yagi was used. The battery-powered radio fired with the antenna pointed at the STP and the link was established. The RF link light came on and the company was able to successfully ping the radio at other end.

Two main reasons contributed to the success of this data link project:

1. Frequency hopping spread spectrum.
2. RF speed is higher than actual throughput. At RF, the higher the throughput, the less the distance a radio will cover and with less throughput the radio can cover longer distances. For example, the FLC 2.4 GHz radio will cover a distance of 8 km with 800 Kbps of throughput and the same radio will cover a distance of 500 m with 11 Mbps data rate.

For Sydney Water it was a great cost-saving exercise. The company saved the cost of installing mast towers by installing the dam site radio in an outdoor kiosk with the Yagi antenna attached to a 1.5 m long bracket mounted on a 2.5 m structure; and at the STP site, an omni directional antenna is attached to a 1.5 m long bracket mounted on a single-storey building.

The West Camden project is one of nearly 20 major projects currently being undertaken by Sydney Water. These include other sewage treatment facilities, the Liverpool to Ashfield pipeline and providing for the population growth to the north-west sector of Sydney.

This feature was written by:

• Asghar Khan - Wired & Wireless Solutions International
  http://www.wwsinternational.com.au

• Richard Camilleri - Sydney Water
  http://www.sydneywater.com.au