Oxygenation of large water bodies
Mankind on planet earth is having to come to terms with the consequences of decades of water pollution. Stuart Piggott, new business manager for Environmental Processes at BOC explains how reacting to this and developing solutions to reverse the deterioration is happening at differing speeds in different geographies and is very much dependant on local situations, in terms of legislative pressure, human and industrial growth and urban regeneration.Historically water pollution shadows both human population growth and industrialisation and can be caused by a combination of diffuse and concentrated nutrient enrichment, for example water runoff from agricultural land or specific effluent discharges. Both will contribute to a gradual (or severe) change to a water body status and result in pollution in rivers and eutrophication in lakes.
The developed world has for the last few decades been tackling this with a reasonable degree of success with multi billion pound capital investment schemes. These investments have been made mainly at the point source of pollution and are part of long-term strategies of gradual improvement, however there are some water quality challenges where this approach is not appropriate and more reactive, and shorter term measures are needed to provide an urgent and rapid solution.
This brief report describes some of the solutions, which BOC has delivered to customers utilising its skills in both dissolving oxygen in and mixing water bodies.
BOC's first foray into water oxygenation was in 1972, when it launched the River Protection Service, comprising 2 systems of pumps, hoses, spargers and oxygen dissolvers. Liquid oxygen would be delivered by lorry direct to the riverbank and RPS operators would deploy the equipment to inject oxygenated water back into the troubled river.
A number of rescues were made including the river Tees, river Lark and Trent. The units were called out more often than not on a Friday afternoon or bank holiday involving much effort to set up. The process was neither manpower nor cost effective yet, no doubt, hundreds of fish were saved.
In 1980, working with the Thames Water Authority, a prototype river oxygenation craft was developed. This was to be deployed on the Thames during or after periods of heavy rain when sudden storm water surges decreased the dissolved oxygen levels in the river. The on board equipment consisted of a Pressure Swing Adsorption plant (PSA) and Vitox® oxygen dissolving equipment. The concept embodied in this barge with a 10 tonne per day oxygenation capacity proved successful and enabled valuable experience to be gained.
The first barge was replaced by a self-powered "Thames Bubbler" with a capacity of 30 tpd in 1988, when it became clear a larger and more flexible system was required. Again BOC working closely with the client to provide oxygen generation plant and dissolving systems. The vessel, capable of a speed of 8 knots, is deployed upstream of any dissolved oxygen sag, and by raising the DO avoids the oxygen depletion which would cause significant fish mortality. Continuous DO monitoring stations allows the Bubbler to be called out and positioned accordingly. The unit can follow the DO sag as it moves, with the tide, up and down the river.
A third vessel "Thames Vitality" was launched in 1999. Although river water quality has improved dramatically since the 1950's when long sections were virtually devoid of Oxygen, sudden storm surges in the ageing sewer system can still cause difficulties in the Thames itself. Thus the mobile Bubbler and Vitality provide reactive systems to ensure the continued improvement of the river Thames.
Experience gained on the river Thames proved useful when in 1986 BOC was employed to design a system to oxygenate a polluted section of sea water which had become isolated when the new Hong Kong airport runway was built out into the bay. The section of water, used as a typhoon shelter, was receiving polluted water from a number of feeder channels and was causing a local nuisance. 6 fixed Vitox systems were designed and installed to mix and oxygenate a 1.5 kilometre body of water 200-m wide.
Moving on up
Fixed systems have also been developed for use on a number of UK rivers in the past, these include the Tyne, Tees, Medway, and Tywi. The systems were designed to add sufficient oxygen to the rivers to maintain healthy conditions for fish movement where tidal conditions exacerbated an existing problem. Annual summer deployment on the Tyne in the late 1990's ensured successful salmonid migration through the river. The systems were of relatively low capital cost and part of long term investment plans in these areas which has now negated their continued use.
Further work was undertaken by BOC in Australia in 1998 and 1999 as part of the Swan Canning Clean-up Programme. The oxygenation trial was conducted on a stretch of river which often suffered from anoxia and phytoplankton blooms in summer.
The factors governing nutrient cycling in aquatic ecosystems are complex but dissolved oxygen levels play an important part in the process. Not only does lack of dissolved oxygen cause release of sediment bound nutrients, but positive dissolved oxygen levels enable the normal nutrient cycling systems to process nutrients more efficiently.
By installing BOC's sidestream oxygen dissolving technology, this double benefit was demonstrated during the 6-month trial on the Canning and proved a simple and rapidly responsive solution to stagnant water systems.
Oxygen is a relatively insoluble gas, which can take considerable power to dissolve into water. BOC has gradually developed the Vitox process over the last 30 years, with each project a new improvement or development has been incorporated. The latest systems incorporate designs, which minimise friction losses, thus reducing specific power requirement and maximising dissolution efficiency. Various patented processes are to be found on board these oxygenation vessels.
China is one of the worlds most rapidly developing economies and Shanghai has been a hive of industrial activity since the 1920's but more recent growth has severely challenged the natural rejuvenation of the area's watercourses. The Suzhou Creek flowing through Shanghai by the end of the 1990's was becoming in need of urgent help. As part of a massive clean up programme and in order to improve the riverside environment in Shanghai, BOC was awarded a contract in 2001 to provide an oxygenation barge.
The contract was the culmination of 18 months work by BOC teams in the UK and China working closely with the Shanghai authorities. The barge is a smaller version of the Thames Bubblers with onboard oxygen generation and state of the art, newly patented, dissolving and mixing systems. The oxygen generation and Vitox dissolution technology was designed and built in the UK and flown to China for installation in the barge. BOC engineers commissioned the Suzhou Creek Bubbler in November 2001 before the official ceremony of handing it over to the Shanghai vice mayor.
The plans to impound Cardiff Bay, initiated in the late 1980's were to have major water quality challenges. Paramount was the need to protect and sustain a healthy migratory fish population. The Cardiff Bay Barrage Act (1993) stipulated that the impounded area of 200 ha of freshwater had to maintain 5mg/l dissolved oxygen concentration at all times.
BOC was involved during the early stages of establishing potential remedies to times of oxygen shortfall which were predicted during still, hot and dry weather. Field trials were undertaken in association with the Water Research Centre in Greenland Docks, London producing valuable data on mixing, oxygenation and the wind effect. During further University research, BOC used computational fluid dynamics to study effects of river flow into the Bay and the effects of depth, wind and temperature on dissolved oxygen levels.
This was used to propose various solutions to Cardiff Bay Development Corporation and ultimately the Cardiff Harbour Authority, culminating with the delivery of a vessel named "Harbour Four". The vessel is equipped with twin Vitox systems capable of dissolving 5 tonnes of oxygen per day. It also has a newly developed and patented Varijet system, which allows the angle of the oxygenated water jets to be changed depending on the depth of surrounding water.
During the summer months, when DO monitoring stations trigger its' deployment, Harbour 4 is manoeuvred within the River channels and around the various locations of the bay to provide the necessary oxygenation and without disturbing the river or bay sediments.
The vessel exceeded all expectations during its' first summer of operation in 2004 and with BOC providing the routine maintenance on the oxygenation system all is set for a hot 2005 summer.
No off the shelf solutions exist for the infinitely variable challenges of urban water regeneration schemes. However, BOC has demonstrated that its understanding of oxygen dissolution into water and the mixing and oxygenation of large water bodies coupled with a willingness to work with clients to evaluate new projects can produce innovative and very effective solutions, which deliver long term value to the customer.
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