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A modern way to deal with two age-old problems

Sewage systems have two inherent problems; they can be the source of unpleasant odours and they are subject to corrosion. The first represents an unpleasant environmental nuisance, while replacing corroded pipes is both time-consuming and costly. These problems are now being addressed in a thoroughly modern way, thanks to Kemira’s H2S-Guard™.

The odour and corrosion problems typical of sewage systems are normally dealt with by using chemicals. Achieving the correct dosage so that it is sufficient to be effective but does not cause other problems has always been difficult, however, and has made chemical usage something of a hit-or-miss affair.

It was against this background that Kemira decided to develop an automatic dosing and control system using dedicated hydrogen sulphide (H2S) measurement units operating in real time. This work resulted in the Kemira H2S-Guard™ system, which measures H2S levels once a minute and sends the data via GPRS technology to a server, where software calculates and adjusts dosing rates to keep gas concentrations at an appropriate level. Data is stored and displayed on the Kemira web portal and can be accessed by anyone with the appropriate authorisation.

The Kemira H2S-Guard™ system can provide a detailed, ongoing analysis of things such as sewage flow rate, the temperature inside pipes, pH values, conductivity, sump pit levels, and chemical dosing rates. The system is supplemented with Kemira’s extensive range of ferric- and nitratebased chemicals, which precipitate sulphides and prevent effluent from becoming septic.

In addition to providing highly accurate online measurement of conditions inside a sewer system and making it possible to ensure the optimal dosing of treatment chemicals, Kemira H2S-Guard™ also saves on manpower because of the easy access to data it offers and the minimal installation and maintenance requirements of the system.

Odour issues and concrete corrosion are two common challenges facing wastewater treatment systems, and can be exacerbated by well-intentioned efforts to reduce water usage. In most cases, they are caused by the same chemical substance, hydrogen sulphide (H2S). Being able to measure and control this gas where it is generated is essential.

Keeping those spikes in check

Kemira H2S-Guard™ can be very useful in dealing with some of the unwelcome side-effects resulting from well-intentioned efforts to save water, particularly the reduction in system efficiency resulting from smaller volumes of flushing water. When flow rates are cut, the concentrations of problematic substances in wastewater increases, triggering a host of microbial reactions that often result in higher odour levels than would be experienced normally and an elevated risk of corrosion.

Kemira has developed H2S-Guard™ to provide a reliable, proactive way of dealing with odour and corrosion problems related to H2S by packing measurement, monitoring, and control capabilities into a single easy-to-use, yet very powerful solution.

Under normal wastewater conditions, where the pH value is around 7, the chemical equilibrium is approximately 50% bisulphide (HS) and 50% dissolved hydrogen sulphide (H2Saq). In turbulent conditions or during aeration, wastewater releases the dissolved gas as free hydrogen sulphide gas. To restore the natural equilibrium, the HS is transformed to H2S.

As little as 5 parts per million (ppm) of H2S is sufficient to generate a distinct smell, while higher levels can be very irritating and even fatal if they reach 500 ppm. These phenomena are also the major cause of concrete corrosion in wastewater networks.

Keeping peak levels of H2S in check – ideally below 2 ppm – is essential for controlling odour problems and for minimising corrosion. Being able to do this calls for detailed and accurate data on what is going on inside a system and the ability to guarantee optimal chemical dosage rates – which is precisely what the Kemira H2S-Guard™ system offers as the world’s most advanced solution of its type. Introduced in 2008, Kemira H2S-Guard™ is already operational in more than 200 municipal and industrial installations around the world.

Kemira is a global chemicals business with annual net sales of some €2.8 billion serving customers in water-intensive industries. Driven by its vision of being a leading water chemistry company, Kemira offers water quality and quantity management solutions designed to improve customers’ energy, water, and raw material efficiency.

> G√ľnter Doppelbauer
(Published in HighTech Finland 2010)