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Thinking

Securing a reliable water source for Paarl’s increasing demands

Meulwater

Brendon Theunissen, Project Manager at Aurecon, discusses how a novel treatment process has provided significant cost savings for Meulwater Water Treatment Works.


Urbanisation and expansion of the Drakenstein Municipality’s largest town, Paarl, 60 km north-east of Cape Town in South Africa, have resulted in a higher demand for water.

Paarl previously purchased approximately 95 per cent of its water supply from the City of Cape Town.

The town also had its own water supply scheme in place, which could have provided up to 25 per cent of the town’s annual water requirements. Due to a lack of treatment facilities, the mountain water source is hardly used at all – during 2005/6 only five per cent of the town’s requirements were supplied locally.

A water supply management study conducted in 2001 found that there was a need for the municipality to secure its own reliable water source.

Apart from the obvious need for water treatment, the report showed how the works could be constructed and operated at a substantial net saving, when compared to the alternative of purchasing the equivalent amount of water.

As the engineering consultant, Aurecon had to overcome significant challenges in providing a solution for the water needs of the Drakenstein Municipality in an extremely sensitive environmental area.

Minimising environmental impact


The plant was designed with careful attention to minimising its visual impact.

The structure sits up to 5.5 m deep into the ground at places and has displaced approximately 1 500 tonnes of granite, almost half of which has been retained on site – either as cladding to the structures or as stonepitching around the site – to allow for the building texture to closely approximate that of its surroundings.

The faces of the structures were staggered to lessen their visual impact and the landscaping of the site was designed to limit the visibility of the site from below.

Direct filtration process


The direct filtration process allows the plant footprint to be minimised, achieving a substantial capital cost saving over conventional treatment options of 15 per cent of the construction costs.

Direct filtration is not common in South Africa, and requires a more careful and specific design, particularly as there is no up-front sedimentation step to reduce the load on the filters. The chemical dosing requirements and flocculation process are different to conventional systems where heavy and large settleable flocs are sought. The design at Meulwater Water Treatment Works facilitates a penetrating depth filtration of sturdier and smaller flocs, resulting in full utilisation of the solids storage capacity within the filter bed.

Coarser, deeper beds


Global trends have seen coarser and deeper beds with higher filtration rates than those typically used in South Africa.

At Meulwater, the filter bed is designed with coarse media at 1.2 mm effective size, and a deep bed of 1.5 m so that longer filter cycles are achievable. The coarser bed provides a higher nominal design filtration rate of 8.5 m/hour, with associated savings in construction costs.

Air scour and sub-fluidisation backwash


Filter cleaning is usually accomplished in South Africa by sequential air scour followed by reverse flow (backwash) to wash out the accumulated solids removed from treated water, which can lead to filter deterioration and costly rehabilitation work.

Combined air scour and subfluidisation velocity backwashing enhances the backwash process. If the appropriate combinations of air and water flow are utilised, as has been done at Meulwater, a collapse-pulsing mechanism occurs in the bed, which is more effective at abrading filter media and loosening accumulated and adhering solids/flocs.

This feature is particularly important for the deep-bed direct filtration plant design at Meulwater, to ensure that the entire depth of the bed is cleaned with minimal loss of water.

Flexible design


The Municipality may in the future consider increasing the annual volume of Berg River water that is pumped into the Nantes Dam to supplement the natural inflow. To allow for this, the filters were designed to accommodate conversion to dissolved air flotation-filtration units (DAFF process) by simple addition of the necessary white-water supply pipework, and provision of white-water production equipment.

The dissolved air flotation process allows for injection of a side-stream into the filters of treated water that has been super-saturated with air at high pressure. A sudden drop to atmospheric pressure at the injection point allows for formation of microbubbles, which then attach to algae and chemically prepared flocs causing these to float to and accumulate at the surface of the water in each filter as a float layer of ‘scum’.

With a relatively small cost and without need for any construction work, an entire phase separation process can be incorporated into the existing plant in the future.

Declining rate hydraulic control


The design of filters with declining rate hydraulic control is an uncommon feature in South African plants but has been shown that it can produce a better filtered water quality and provides capital cost savings.

The design of the process requires more comprehensive hydraulic modelling than for conventional systems, as the flow is not distributed evenly amongst the duty filters. Instead, each filter transmits a flow proportional to the total head provided and in equilibrium with the degree of clogging since the last backwash. The fact that a higher flow is not forced through the more ‘dirty’ filters prevents early breakthrough of attached flocs, while the cleaner filters transmit a controlled yet higher flow. Declining rate filters require a lower clogging headloss (about 30 per cent saving), as the energy available to the cleaner filters is not ‘lost’ to a modulating valve as for conventional systems.

Enhanced dosing control


The plant chemical coagulant dosing system incorporates an online mass flow metering system, which measures the actual mass of active ‘ingredient’ in the coagulant dosed.

The operators simply select the actual dose required and the automatic control system adjusts the dosing pumps to achieve that target whilst allowing for the actual plant flow.

This system allows greater accuracy of dosing control, with improved treated water quality and potentially significant savings to the annual chemical cost.

Successful delivery of a novel solution


The 8 M?/d Meulwater Water Treatment Works was successfully commissioned in July 2012.

The enhanced process design deviated from conventional plants and was tailored specifically for the actual raw water to be treated.

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