Technology offers clarifier solutions
An innovative new gravity settling system aims to overcome current clarifier problems
One of the most widely-used processes in the water and wastewater treatment industries is natural gravity settling. Vast numbers of clarifiers operate in the manufacturing, process and service industries, but although they perform reasonably well, they are not as efficient as they could be.
This presents a growing industrial challenge in view of the fact that water usage is increasing, water quality standards are getting higher and water companies are increasingly refusing to treat discharges whose quality exceeds consent levels. Greater efficiencies are essential. Fortunately, an innovative gravity settling system that is being launched is capable of achieving significantly higher efficiencies than at present. It is being sold under the trade name Voluflow, is simple and robust, has no moving parts and can be supplied in stand-alone form or as a retrofit upgrade to fit inside existing clarifiers.
Current clarifier design problems and the Voluflow solution are described below. It has been known for more than a century that clarifier performance could be improved significantly if all portions of a suspension being settled took the same time to go from inlet to outlet. In other words, maximum efficiency would come with equal treatment for every drop. For this to happen, uniform forward-going flow velocities are needed at all cross-sections throughout the clarifier. Under these ideal flow conditions, nothing would flow through so quickly that sedimentation was incomplete, or so slowly that valuable time and space inside the clarifier was being wasted.
In practice, the uniform flow conditions that are required for maximising efficiency never actually happen, even in the best designed traditional units. Adverse flow conditions such as short-circuiting, turbulence, large-scale recirculation and stagnant zones occur. These flow conditions and resultant carry-over of solids arise from the momentum of flowing liquids, which keep on going until diverted by an obstacle, whereupon new flow directions develop and again persist, and so on.
The final effluent is therefore a mixture of many different flow pathways, some of which are heavily contaminated, hence poor effluent quality. Conventional circular clarifier designs actually encourage these flow patterns as a result of the influent liquors being directed down to the bottom of the clarifier. The idea is to generate an even distribution of slow flows upwards and outwards throughout the clarifier, which happens to some extent. At the same time, however, much faster flows fan out across the bottom of the tank towards the outer wall, and up the sides.
On reaching the top surface, some of the water flows over the weir, while the rest returns to the centre and down again. Large-scale circulation is established, driven by the continuous influent flows that are streaming down to the bottom of the clarifier. Large stagnant zones also form inside the circulation streams, where there is plenty of time for settling to be completed, after which the space is under-utilised - stagnant zones spell inefficiency.
Short-circuiting also occurs when weirs are not level and when parts of the weir become blocked with debris. If flow rates increase beyond the design value, as can happen all too often in practice, these problems become even more serious and lead to such effects as scour, and so to further effluent contamination. Finally, when surging occurs, the resulting high velocity streaming flows dig into the light, settled sludge, forcing it up bodily to the overflow weir, causing catastrophic contamination.
To accommodate all these problems with conventional clarifiers, it has become established practice to oversize equipment. This increases equipment and operating costs and the larger footprint wastes valuable plant space. However, even when conventional design clarifiers are oversized, they are still sensitive to the effects of influent flow rate variation, especially surging. Oversizing sometimes includes deepening of clarifiers, to avoid scouring effects from the downward stream of incoming liquor and to facilitate higher surface loadings.
This is practiced more in the US than in Europe, but it is an expensive option that is not needed with Voluflow technology. As already indicated, the need for uniform flow velocity fronts in settling tanks has long been recognised but progress has been slow. Physicist Doctor Bill Foster of Fostech, a process development company in Northern Ireland, has systematically addressed this problem for more than ten years. The outcome is the highly efficient Voluflow system, which is being tailored for the market by water filtration and treatment specialist Atkins Fulford.All in the name
The name Voluflow reflects the volute shape of the vertical scrolled baffle that distributes flow uniformly. The Voluflow clarifier design is unique. Flow velocities are very low, there are no swirling flows and no centrifugal forces. The system eliminates adverse flow patterns by first stilling the flow in a special inlet zone, then slowly distributing the suspension evenly throughout the full depth and width of the clarifier channel, after which slow, horizontal, forward-going flow patterns prevail throughout.
Cross-flow settling predominates so solids settle freely without any counter-current upward flows, allowing surface loading to be increased. The conditions are as close to those that occur in static settling as it is possible to be, the nearest approach to batch settling, continuously. The latest Voluflow unit to be installed by Atkins Fulford was at G's Fresh Beetroot factory in March, Cambridgeshire. Liquors from the beetroot skinning process pass through a drum separator to remove coarse pieces and then into the central distributor of a 4m diameter, 600 conical-bottomed clarifier.
Before the Voluflow retrofit was installed, the effluent quality varied considerably, mostly below the consent level of 1,500mg/l, but sometimes above. To achieve better performance and to eliminate overshoot, a Voluflow retrofit was installed. It took only 4h to remove the existing gantry and lower in and secure the retrofit complete with new cross gantry. The upgrade was an immediate success with effluent solids running at 500-1,000mg/l, and no sign of exceeding the consent level.
Site engineer Fred Foreman said: "It is G's Fresh Beetroot policy to seek and secure environmental benefits. The Voluflow clarifier ensures we collect all our beetroot debris for injecting into sandy land to improve it. This is much better than letting it go to drain to be mixed with environmentally-unfriendly sludge. Voluflow operating costs are likely to be less than before and consent levels will not be exceeded."
Secondary treatment clarifiers have also been installed in factories. Three years ago, managing director Tommy Thompson of TMC Dairies near Strabane in Northern Ireland made a bold move and had a Voluflow retrofitted into an underperforming 6m diameter activated sludge clarifier. As a result, throughput and effluent quality have consistently improved and sludge lift never occurs. Derek Hetherington, who runs the plant said: "Since installing the Voluflow, this has become a very good clarifier - 100%."Measured success
To quantify the improvements, a performance evaluation was undertaken by Questor, an off-shoot of the Queen's University, Belfast. One of Questor's conclusions was: "The smaller footprint of the settler and the loading rates that have been achieved show the technology is a substantial improvement on the existing settler configurations at this site." Another user, Cambrex, a pharmaceutical company in Cork, Republic of Ireland, commissioned a new-build 4.2m diameter Voluflow activated sludge clarifier last February. From day one, performance improved significantly in all respects.
It successfully handles high influent suspended solids concentrations, even 15,000mg/l. The unit is surge-tolerant, sludge level control is facilitated by a built-in sight glass and effluent suspended solids are always well below consent level. Engineer in charge, Jonathan Sowerbutts confidently recommended the system and said: "The Voluflow clarifier is an innovative design that performs far in excess of others of the same footprint. The after-sales service is also excellent."
The end result of Voluflow technology - significantly improved performance and easier process control - is now available in a range of applications. Atkins Fulford focussed initially on the difficult area of activated sludge clarification but can also provide free-standing or retrofit equipment for other applications such as grit traps, oil interceptors, primary settling, and even storm tank and lagoon settling.