Hydraulic Flocculation Research Articles

The impact of physico-chemical water treatment on a novel flotation/filtration process

Recent research explored the application of mechanical flocculation ahead of a novel flotation/filtration process (Counter Current Dissolved Air Flotation/Filtration - COCO DAFFTM) and compared it to the process performance seen when hydraulic flocculation was used. The energy input to the system, expressed in terms of the mean velocity gradient, G (s−1), and retention time, t, were used as a guideline for comparison between the two types of flocculation. Generally, better COCO DAFFTM performance resulted from using mechanical flocculation as against that using hydraulic flocculation. High G values (in excess of 80 s−1) were found to give best results, as opposed to values of 30-60 s−1 utilised ahead of conventional treatment. The effect of flocculation contact time on the COCO DAFFTM process was also studied. Current design practice typically utilises 20-30 minutes flocculation contact time. It was found that process performance, in terms of water quality, did not deteriorate excessively if the contact time was reduced to as little as 2 minutes, but filter run times were found to be proportional to contact time.

Field trials of appropriate hydraulic flocculation processes

Hydraulic flocculation has been used for water treatment throughout the world for many years and is particularly well suited for developing countries. Two types of flocculator are described, a horizontal-flow channel system, for which aspects such as retention time, channel slope, rapid mixing methods and baffle insertion were assessed and a contact flocculation-filter (CFF). Pilot plant tests were carried out over two years at Thyolo Water Treatment Works in Malawi using river water with crushed seed kernels from the indigenous tree Moringa oleifera Lam. ( M. oleifera) or aluminium sulphate as coagulant. Both flocculation systems were able to reduce turbidity to below the World Health Organisation's guideline value of 5 NTU for drinking water from raw water with average turbidities during test runs ranging from 15–5600 NTU. The CFF method was particularly effective at low turbidities where <1 NTU was attained at low dosages (<25 mg/l) of M. oleifera. “Flash” mixing of coagulant using a simple equivalent of an in-line vortex mixer was shown to give more efficient treatment than mixing by alternative methods. M. oleifera as coagulant is shown to be feasible for treating river water for communities up to small town size and its production close to treatment plants would obviate importation of chemicals thus saving on foreign exchange, provide economic development of rural communities and reduce potential health hazards from chemical overdosing.

The impact of physico-chemical water treatment on a novel flotation/filtration process

Recent research explored the application of mechanical flocculation ahead of a novel flotation/filtration process (Counter Current Dissolved Air Flotation/Filtration - COCO DAFF™) and compared it to the process performance seen when hydraulic flocculation was used. The energy input to the system, expressed in terms of the mean velocity gradient, G (s−1), and retention time, t, were used as a guideline for comparison between the two types of flocculation. Generally, better COCO DAFF™ performance resulted from using mechanical flocculation as against that using hydraulic flocculation. High G values (in excess of 80 s−1) were found to give best results, as opposed to values of 30–60 s−1 utilised ahead of conventional treatment. The effect of flocculation contact time on the COCO DAFF™ process was also studied. Current design practice typically utilises 20–30 minutes flocculation contact time. It was found that process performance, in terms of water quality, did not deteriorate excessively if the contact time was reduced to as little as 2 minutes, but filter run times were found to be proportional to contact time.

Original Article

Hydraulic flocculators, despite their superior plug-flow properties, negligible energy requirements and robust simplicity, are commonly perceived as being inflexible. Once built, it is argued, they do not provide the operational flexibility to compensate for flow fluctuations or changing raw water characteristics. The design procedure developed in this paper demonstrates the contrary, namely that hydraulic flocculators can be designed to satisfactorily cope with tapered velocity gradients, flow variations and water quality variations. The procedure is based on a two-step approach; the horizontal layout is first determined with average values for velocity gradient and water depth; then only the floor slope and exact water depths are determined to enable satisfactory flocculation for all design scenarios.

Design of around-the-end hydraulic flocculators

Hydraulic flocculators, despite their superior plug-flow properties, negligible energy requirements and robust simplicity, are commonly perceived as being inflexible. Once built, it is argued, they do not provide the operational flexibility to compensate for flow fluctuations or changing raw water characteristics. The design procedure developed in this paper demonstrates the contrary, namely that hydraulic flocculators can be designed to satisfactorily cope with tapered velocity gradients, flow variations and water quality variations. The procedure is based on a two-step approach; the horizontal layout is first determined with average values for velocity gradient and water depth; then only the floor slope and exact water depths are determined to enable satisfactory flocculation for all design scenarios.