Wired for water07 June 2011

Technologies available to cut the cost of treating waste water, by improving efficiency and obviating practical problems, are many and various. Dr Tom Shelley explains

All municipal and much industrial water treatment centres on harnessing bacteria to break down unwanted organics, and supplying oxygen to help them do so. The trick is to maximise the efficiency of the process, for example, by improving closed-loop plant control – or by using reed beds to do the work.
Reed bed advocates make the point that plants are powered by sunlight, so require no additional energy source – just, possibly, pumps to deliver waste water and remove processed water, if gravity is not enough. They also point to reed beds' longevity.

"I have evidence that reeds can continue to work effectively after more than 120 years," says Melvyn Rutter, of Yes Reedbeds in Yorkshire. And he explains: "My reed beds create aerobic conditions for bacteria to break down the biological and mineral pollutants in waste water. It should not be a surprise that this living environment has done this kind of work for more than 400 million years. Reedbeds are simply nature boxed up. In a sewage works you expect to see the clinker beds; reed beds use the same principle, but with less concrete and computers."

In fact, Yes Reedbeds have been built to filter domestic sewage, ochre mine waters, brewery wastes, vegetable waste waters, carbohydrates starches and colloidal hydrocarbons, ammonia, nitrates, phosphates and other pollutants. And they can also cut the costs of treatment. "One recent system was built at a cost of £18,000, but this saved the company £30,000 per year over other disposal methods," says Rutter. "There is a serious future for reed beds and other ecological solutions to human problems of pollution. Ecological solutions allow human development in balance with the natural world."

The only downside is the amount of space they require, which makes them fine for rural or semi rural environments, but not urban environment, which will continue to require conventional aerated tanks.

Moving on to conventional water treatment plant and systems, the big deal remains cutting energy consumption and reducing maintenance requirements. Faster response and more accurate instrumentation and controls, for example, allow aeration energy to be reduced for a given dissolved oxygen requirement. However, until a few years ago, one barrier was with electrochemical dissolved oxygen sensors, which tended to have problems and require frequent recalibration.

One solution was Hach Lange's robust LDO (luminescent dissolved oxygen) sensors – patented technology unveiled back in 2002 that has since become fairly standard. Its probe tips are coated with a luminophore – a luminescent material excited by blue light from an internal LED. As it relaxes, the material emits red light in relation to dissolved oxygen, with an internal red LED providing a reference measurement to maintain calibration. Importantly, it also only requires cleaning once every six months.

But the latest real time sensing and control systems from Hach Lange do not only measure dissolved oxygen, but also ammonium and nitrate – and with considerable success. Test results on two identical activated sludge process lines at a 250,000 population equivalent works in the UK are said to have shown that a real time control system aimed at cutting ammonium content actually reduced methanol consumption by 50% and air flow by 20%. That system has now been in operation for more than a year.

Incidentally, the company also offers real time control technology for removing phosphates, much of which comes from synthetic detergents. These have to be removed by chemical precipitation to avoid algae growth and resulting low oxygen in the effluent. A number of processes are in use based on various chemistries, but systems using Hach Lang equipment have been shown to greatly reduce chemical consumption. Payback times are also short: Hach Lange cites an unnamed UK works said to have saved 37% of ferric sulphate and 57% of caustic chemical costs. The firm also claims that a plant in Italy achieved 50% cost savings, representing a payback of just seven months.

Meanwhile, sensing and managing toxic and flammable gases, such as methane in waste water treatments plants, has also improved. Most methane on modern plants is collected to provide fuel for generators to power plant equipment, but it's not always that simple.

At the Dokhaven waste water treatment plant in Rotterdam, for example, which is underground in the centre of the city, a ventilated system maintains air pressure in the processing chambers lower than that in the tunnels. That prevents any escape of contaminated air, which is instead fed back after passing through filter systems. All well and good, but ensuring safety during the first biological treatment stage (when oxygen is added to the waste water) meant installing 12 explosion-protected LEL (lower explosion limit) gas detectors.

These have 24V dc, 3.5W three-pole connection, as well as 4-20mA signal outputs, and maintenance engineers would normally require hot work permits for servicing. In this case, plant engineers solved that problem by using eXLink plug-in connectors, supplied by Cooper Crouse-Hinds. These allow connection and disconnection in explosive atmospheres without tools and without having to isolate the apparatus from the mains or disconnect the terminals. That's important, because if the detectors fail, the plant is automatically shut down.

But different challenges can be found on plant used for treating industrial waste water streams. In the food and drink industry, for instance, Siltbuster Process Solutions solved the problem of removing solids, fats, oil and grease using what it terms 'packaged lamella dissolved air flotation' units. With this process, air is dissolved in the water under pressure, and is then released as very fine bubbles as pressure reduces. Organic particulates then attach to the air bubbles as they rise to the surface, where they are collected, thickened and removed by a motorised scraper.

Users include Premier Foods' Ambrosia Creamery in Lifton, Devon, where a unit with a lamella plate area of 100m2 reduced the chemical oxygen demand loading on the biological treatment stage by an impressive 30%.

Interestingly, the idea of floating particles attached to air bubbles comes from minerals processing, where effluent tailings can pose even greater processing problems than those typically experienced in the food industries. Apart from the waste from gold extraction, using cyanides, one of the most challenging effluent treatment problems in the world is that associated with the Berkeley Pit in Butte, Montana in the USA.

The more than 150 billion litres of water from this former copper mine are so toxic that birds landing on the lake are soon fatally poisoned. The water level is rising and, in order to prevent contamination of local sources of water, a water treatment plant has been constructed that can process up to 27 million litres every day.

The mine waters are acid, and the treatment consists of a two-stage lime (calcium hydroxide) precipitation process in combination with a high density solids phase. Lime, aeration and polymer addition remove the metals from the waters, and these are then cast back into the pit.

Dr Tom Shelley

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Eaton Crouse-Hinds
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