Treating seasonal algal blooms

Seasonal algal blooms can cause major problems in water treatment works, particularly those where treatment is by direct filtration. Counter Current Dissolved Air Flotation (COCODAF) is now established as a high-rate process for the treatment of low-turbidity, coloured or algal-laden water writes Paterson Candy.


Seasonal algal blooms on storage reservoirs can cause major problems in

water treatment works, particularly those where treatment is by direct

filtration. High algal loadings can cause a rapid increase in filter

headloss and a consequent reduction in filter run times to just a few hours.

Due to eutrophication of many reservoir sources, the trend is for algal

loads to increase, and existing single stage (direct filtration) plants are

becoming unable to operate reliably during summer months.

In the COCODAFF process, flocculated water is distributed evenly near the

surface of the COCODAFF units by a system of submerged laterals and

distribution cones and flows down through a rising air-bubble blanket to the

filter below. The flow of flocculated water is counter-current to the rising

air blanket, promoting greater bubble-particle interaction and enhancing

floc capture efficiency compared to a conventional co-current DAF process.

The micro-bubbles attach themselves to the floc particles and float to the

surface. The resulting sludge blanket, supported by the air blanket below,

is exceptionally stable and is removed readily by hydraulic means. Any

sludge that is knocked down during desludging must pass through the air

blanket again before reaching the filter media.

On an existing direct-filtration treatment plant, upgrading the process by

the construction of a new DAF or COCODAFF plant may not always be feasible

due to space constraints or the complexity of connecting with existing

processes. The cost of constructing a stand-alone pre-treatment stage may

also be difficult to justify. Using in-house computational fluid dynamics

(CFD) and engineering expertise, Paterson Candy has adapted the COCODAFF

technology for installation (retrofitting) into existing filter tanks. This

means that existing direct filtration treatment plants can be upgraded to

include the benefits of flotation without the need to construct additional

process units. Installation of COCODAFF technology can enhance plant

performance and in some cases may allow plant throughput to be increased.

Any dissolved air flotation process requires additional flocculation time,

compared with a direct filtration process, necessitating the construction of

new flocculators. For a conventional DAF process, the flocculators are

typically sized to provide up to 30 minutes flocculation time. With

COCODAFF, the flocculator volume is reduced to provide 15 minutes

flocculation time as some flocculation is deemed to take place within the

counter-current air blanket. For COCODAFF, Paterson Candy offers hydraulic

flocculation yielding operational cost benefits and reduced maintenance

requirements compared to conventional mechanical flocculators.

A major benefit is enhanced algae removal efficiency leading to maintenance

of filter run times during seasonal algal blooms. Trials have shown that

during algal blooms, a dual media COCODAFF process is able to maintain

filter run times in excess of 24 hours, while corresponding dual media

filters, operating in direct filtration mode and at reduced filtration rates

are struggling to achieve run times of four hours. In this situation, there

is a reduction in the amount of dirty washwater generated with the COCODAFF

process and a saving in operating costs for the wastewater handling plant.

The process offers operational flexibility. During periods of good quality

raw water, the system may be turned off and the plant operated in direct

filtration mode, minimising operating costs. During periods of seasonal

algal blooms or increased raw water colour, the COCODAFF process can be

quickly brought into service without interrupting the operation of the

filters.

COCODAFF technology can be engineered to fit a wide range of filter designs.

Conversion of existing filters to the process involves the installation of a

new inlet channel which, depending on the layout and size of the filter, may

be positioned across the centre or along a side wall. Inlet cones, the

design of which have been optimised using CFD to maximise inlet distribution

while minimising headloss, are arranged on a number of distribution laterals

fed from the inlet channel. The recycle distribution system is suspended

from the inlet distribution pipework.

To facilitate the hydraulic desludge, a flushing water channel and a sludge

collection channel are installed along opposite walls of the filter. During

a desludge, a sludge outlet valve opens and the sludge blanket is pulled

over the desludge weir. Simultaneously, water fills the flushing water

channel and overflows the ski-jump shaped weir, pushing the sludge blanket

towards the desludge weir. The desludge is carried out automatically. The

interval between desludges can be adjusted to suit changing raw water

conditions.

The installation of COCODAFF technology to existing filters can enhance

process performance, particularly when algal blooms pose a problem, and lead

to a flexible and more robust treatment process solution.


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