Keep it clean
Ever wondered what happens to industrial wastewater after it goes down the drains? EBM looks at one sewage company's approach to effluent treatmentIn the old days of sewage treatment, finding out whether there had been a consent breach was a thankless task. Teams from the treatment company and the Environment Agency had to visit each site, take samples, send them for analysis then wait 10 days for the results.
Not only was it incredibly labour-intensive, it also meant slow response times leading to high levels of environmental damage. "It meant that the Agency took a lot longer to inform us that there were problems," says Andrew Hunt, Southern Water's environmental services manager.
Thankfully, technology has moved on. About two years ago, Southern Water developed and installed a computerised system for effluent monitoring which reduces the risk of consent breaches at 180 of its wastewater treatment works - representing over 85% of the flow from 4m customers.How it works
All sites are equipped with specially adapted turbidity monitors. These provide online data to a 24-hour control centre and a stand-alone telemetry archive. When a breach is about to occur, the control centre sends an email alert to operators for an immediate response. The monitors sit in the final effluent chamber and measure discharge cloudiness.
A bespoke computer application analyses archived data overnight, and sends email warnings to technical personnel the next morning. The latter application, proven over the past two years, has initiated over 1,400 process queries, resulting in a significant reduction in consent breaches.
"What we are trying to guard against are any sudden problems," says Hunt. "The system looks for any change from a nice steady trend to an increase in effluent turbidity."
It sounds simple but a number of technical challenges had to be solved before the system ran smoothly. There were two major hurdles - the fouling of the meter optics and ascertaining the relationship between turbidity and the consented parameters, biological oxygen demand (BOD) and suspended solids (SS).
Southern Water overcame the fouling of the optics by using a combination of wipers and blasts of compressed air. It also discovered that the relationship between turbidity, SS and BOD are different at each works. With this knowledge, alarms are set at site-specific levels.
With other data such as flow rates, temperature, and ammonia and phosphate levels also stored in the telemetry archive, turbidity can be aggregated against timescales as graphs or spreadsheets, ranging from a few hours to several years, using specially created software. This allows technical staff to investigate problems and advise operational personnel on how best to maintain the works under their control.Benefits
Aside from the reduction in labour hours, there are obvious benefits to the technology. One is the speed of response. The alarm trigger-point is set below the breach level and the early warning allows the company to respond to the changing data trends and sort the problem out before a consent is breached.
This has led to a better relationship with the Agency and Ofwat, the water industry regulator. Ofwat, under the terms of the Water Resource Act 1991 and the Urban Wastewater Treatment Directive 2000, looks at the compliance record of the water companies over a whole year and can issue fines.
Consent breaches are now monitored daily, whereas under the old system it was only 11 or 12 times a year. "This means we can check that there is no chance that the system is going to fail when the Environment Agency turns up to take a sample," says Hunt.
It has also allowed Southern Water to challenge some of the sample results produced by the Agency when it thinks that a mistake has been made - which can happen four or five times a year.
Maintenance costs are reduced - an annual calibration visit is usually all that is required. It also targets the attention of scientists on the sites that need the most help. Their workload is managed by the consent alarms and sites with recurring problems can easily be identified.
At a cost of between £5,000-£7,500/unit/site, Southern Water feels it has made a sound investment. The number of breaches has dropped while the number of enquiries - email warnings - has risen. "That's exactly the relationship you would want to see and one of the main reasons for putting the system in," Hunt says. For customers it means they can pump away industrial sludge with confidence.