Odour control in the Kingdom

Carbon adsorbers installed by Siemens at a Saudi wastewater treatment plant.
[More Images]

By Georgios Ioannou

Georgios Ioannou, odour control technical sales manager at Siemens Water Technologies, examines the challenges in the Saudi water market

Odour control in Saudi sewage treatment plants has become more of a concern in the last decade. In previous years, this issue was considered more of a luxury rather than a necessity. Plants were located further out of town and, as a result, odours were not as noticeable. But odour complaints are now rising as population growth near existing plants and outside temperatures are increasing.

In the past, people did not understand that odours were not merely olfactory nuisances but rather that higher levels of odorous chemical compounds could produce hazardous gases. Hydrogen sulfide (H2S), for instance, can cause injury or death at 100 parts per million (ppm) or greater. Odours may also stem from organic sulfur compounds (dimethyl sulfide, dimethyl disulfide, mercaptans, carbonyl sulfide, or carbon disulfide); nitrogen compounds (ammonia and amines); other volatile organic compounds (VOCs: aldehydes and ketones); and fatty acids.

Today, odour control is generally considered an essential process in municipal and industrial wastewater treatment plant design. The Saudi government has begun requiring that newly built plants implement odour control for a number of reasons. A key concern is that when H2S is oxidised by bacteria, it forms sulfuric acid, which is able to corrode concrete sewer pipes and
metal structures at wastewater treatment plants.

Story continues below

Advertisement

FEATURED COMMENT Please click here to comment on this article

There are two types of odour control treatment: liquid- and vapour-phase. Whichever chemical compounds are causing the odour determine the best chemical, physical, or biological means of eliminating those chemicals or rendering them “odourless.” The odour-causing compound, coupled with the point of origin, is important in selecting the right treatment method. Costs, operational ease, and effectiveness should also be considered when assessing the correct technique for a particular application.

Liquid-phase treatment is typically employed in wastewater collection systems or after treatment at the biosolids stage. This type of treatment involves adding a compound directly into the wastewater to either control the formation of odour-causing compounds biologically and/or chemically or to react with odour compounds once they are formed.

The four main types of liquid-phase treatment include: nitrates, anthraquinone, precipitants (iron salts), and oxidisers (like hydrogen peroxide). When the cost for dealing with odour in this way becomes too high, the cost of capital equipment becomes more cost-effective.

Vapour-phase odour control treatment differs from its liquid-phase cousin in that whereas the latter treats the entire wastewater flow, the former contains and treats odorous air that is released from a given process or area.

Odorous sources such as wet wells, headworks, clarifiers, equalisation basins, sludge tanks, digesters, dewatering areas, and other treatment plant processes are covered, with a system of ducts and fans removing the odorous vapors from these areas. Air is then pulled through a tank that contains some type of media to trap the odorous contaminant before discharging the air stream into the surrounding atmosphere. This capital equipment falls under one of three primary categories: chemical scrubbers; bioscrubbers and biofiltration; and activated carbon systems.

Chemical Scrubbing

In Wadi Al Dawasir, for instance, Siemens has specified a chemical scrubber to meet the Ministry of Water and Electricity’s odour control needs. The Ministry needed an odour-control technology that could fit in a small space and respond immediately to fluctuating loads with high removal efficiencies in excess of 99%.

The Ministry opted to install a patented LO/PRO® multi-stage scrubber system that can target a range of compounds in a single scrubber system. This multichemistry system will reduce chemical costs to less than half that required by conventional packed tower scrubbers.

Chemical scrubbers are the most flexible, adaptable and reliable vapour-phase odour control technology that can be used to treat virtually any water-soluble contaminant. Using a liquid to separate gaseous or particulate contaminants from a gas stream, multi-stage scrubbers can treat several odour compounds such as H2S and ammonia simultaneously. Compared to other vapour-phase odour control technologies, scrubbers are less sensitive to variations in actual versus design H2S loadings. Despite their small footprint, chemical scrubbers can handle air velocities of 2.5 m/s in a single vessel. Although they can be maintenance-intensive and need chemical handling, the systems are easy to install at mild capital cost.