Could combined UV and ozone be the future of odour control?
Hanovia has developed an odour control system based on ozonation and UV treatment. Following a successful installation in Guernsey, wider application looks likely as the market for compact odour control systems expands.The need to control odours from STWs is becoming increasingly common as environmental standards become stricter, the urban population grows and more plants are built which carry out highly intensive treatment processes in a small area.
Principle and design
The same principle which has been applied to breaking down total organic carbon (TOC) in process water is suitable for breaking down the organic compounds found in foul air.
The odorous compounds generated by sewage treatment are generally acidic. Typical compounds include organic sulphides such as H2S and volatile organic acids, such as butyric, propionic and acetic acid.
These fatty acids often inhibit the formation of ammonia, amines and volatile organics such as indole and sketole. When present, these latter species are a particular nuisance due to their persistence.
Hanovia designed the lamps for the Odex system using a similar program to that used for the development of its SuperTOC lamps, used for TOC reduction in water. Undesirable compounds can be destroyed by UV light at wavelengths close to that of the compounds’ absorption spectra.
Lamps in the Odex system produce UV light which breaks down sulphides and mercaptans in the following way:
Hydrogen sulphide (H2S) + UV light energy = H + SH
Methyl mercaptan (CH3SH) + UV light energy = CH3S + S
These lamps do not break down every common odorous compound. To break down amines and ammonia, UV lamps which create ozone are necessary.
Work by Hann in 1983 had suggested that the primary mechanism of odour control by ozone was one of masking, rather than oxidation. And Tuggle (1972) showed that at 39oC, only 37% of the H2S was oxidised in 60 seconds when using an 03:H2S ratio of 1:1.
Hann found in a study of STWs in Orange County, New Jersey that ozone alone could not be relied upon to cope with the variable nature of wastewater odours, and that downwind depletion of the ozone allowed the return of the odour nuisance. Most of the ozone equipment was decommissioned as a result.
However, Hanovia now argues that a very cost-effective odour control process can be achieved using a combination of ozone and UV.
Amines (R = (CH3)3N) are easily oxidised by ozone:
R + O3(ozone) = R3NO (amine oxide) + O2
The Odex system consists of a series of lamps which produce both ozone and UV and then destroy the ozone in the correct sequence.
The airtight treatment unit (known as the plenum) is constantly maintained under a negative pressure, to draw air in through an intake.
Blowers maintain the pressure difference, air flow and force the treated air to leave via a vent stack.
The ozone producing lamp assemblies are located so as to introduce ozone into the receiving section of the plenum.
The ozone-producing lamps discharge UV light at 185nm, which creates ozone when in contact with air. Compressed air is used to purge the quartz sleeves that contain the lamps and provide the ozone.
UV lamps of a different frequency produce copious amounts of UV of wavelengths which are suitable for destroying sulphides.
The rack-mounted UV lamp assemblies are mounted inside the plenum across the airflow.
As the airflow is pulled around to the discharge portion of the plenum, further racks of UV lamps are used to ensure that oxidation is complete.
The ozone (O3) is then broken down into harmless oxygen (O2) by a final rack of UV lamps at 254nm. This ensures that no ozone is ever released to the atmosphere.
The process is centrally controlled and is normally kept in a feed forward mode via telemetry from H2S sensors positioned upstream of the plenum. It is kept in feedback mode by ozone sensors which are installed in the vent stack.
Additional lamps can be set to switch on during peak loads or if a lamp fails. They can also be switched off when not required, to save energy.
Lamp response is within 0.5s of any given input signal. In this way, the UV output can easily be optimised against the receiving load.
System maintenance consists of an annual lamp change and a sleeve change every two years. Individual lamp life is guaranteed at 8000hrs.
The process is said to be unaffected by variations in temperature and even when operating at maximum output, is very quiet.
Case study - Guernsey
The Channel Islands seem to have become a testing ground for compact sewage treatment technologies. One of the first UV installations for wastewater disinfection was on the neighbouring island of Jersey.
Hanovia’s Odex system for odour control has been installed at the Fountain Vinery STW on Guernsey.
Here the odour nuisance was generated by the pumping of waste from septic tanks to a wet well. The waste is pumped out in 5mins, up to 40 times per day.
Residents close to the site had complained about the odour which was shown to contain up to 500ppm of H2S below the wet well covers.
The Odex system installed has 6 lamps, and has a total power consumption of 1.5kW, including the motive air compressor. The system was prewired and built inside a GRP plenum by Hanovia.
According to Hanovia’s managing director John McClean: “When a biological system was considered for this site, a power consumption of 2.8kW was estimated - the Odex system uses just 1.5kW for an airflow of 500m3/hr.”
Data collected with Exotox and Crowcon gas detectors showed a 98% reduction in the H2S load from 480 ppm to 11.3ppm, using all six lamps. One UV lamp was sufficient to achieve an HS reduction from 480ppm to 57.8ppm.
Hanovia says the first installation of the system on the mainland is now expected, following interest from several major companies.
Nigel Horan, an environmental consultant at the University of Leeds said: “This system sounds like it might be a lot more sensible than some of the more quirky technologies that I have seen coming out.
“At the moment, there are about 15 new odour control technologies appearing every year.”
This article is based on a technical paper presented by Mr McClean at the recent CIWEM - IAWQ conference.