- 27 July 2006 -

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Clarification: the rapid-response mobile option

What happens if you suffer clarifier outage? Chuck McCloskey of Siemens Water Technologies' mobile and on-site services says that process downtime doesn't have to be the only option.

Mobile water treatment has been around for many years, providing temporary and emergency water to industrial and municipal customers throughout North America - and internationally.

While mobile clarification is a much newer technology, since its introduction in 2002 the trend toward using mobile clarification has increased, and so have the markets that use this technology. End users can now be confident that they do not need to curtail production or operations due to an emergency or planned clarifier outage.

Siemens Water Technologies was the first to develop the mobile clarifier four years ago. It did this by taking an Actiflo compact, conventional-type water clarification system developed by water equipment supplier Kruger, and transforming it into a self-contained, trailer-mounted clarification system capable of treating water with a wide range of turbidity (20 to >1,000 NTU).

The clarification process

The Actiflo process uses microsand as a seed to enhance floc formation and increase the settling rate, and in many cases using microsand leads to the production of higher quality effluent - than that produced by other types of clarification systems. It is particularly suitable for treating any raw water that is traditionally difficult to treat - such as water with very high or low turbidity, high colour/TOC, cold temperatures or high algae. It provides clear effluent water, even under the most demanding raw water conditions. This process gives a turbidity of the settled water typically less than 2.0 NTU (suspended solids below 5 milligrams per litre).

How does the clarifier work?

Raw water enters the coagulation tank, where chemical coagulant (alum or ferric) is added to destabilise the suspended solids and colloidal matter in the influent stream. Mixing is provided at this stage to thoroughly incorporate the coagulant into the raw water. The coagulated water passes into the injection tank, where polymer and microsand are added to initiate floc formation. In this tank, the polymer acts as a bonding agent between the microsand grains and the coagulated matter, forming flocculated particles that are many times the size of the individual coagulated matter. At a nominal 100 micron diameter, the microsand is much larger and denser than the lighter coagulated particles, and the larger sand grains, covered with polymer chains, act as floc scavengers to collect and capture floc particles in the flow stream. The larger sand grains increase the occurrence of floc collision, resulting in higher water clarity.

Next, the water passes into the maturation tank, where gentler mixing forms polymer bridges between the microsand and the destabilised suspended solids. The fully-formed, ballasted flocs leave the maturation tank and enter the settling tank, where the weighted flocs settle to the bottom of the clarifier. The clarified water flows upward through the lamella tubes in the clarifier zone, and exits the system via a series of collection troughs or weirs for subsequent treatment or distribution.

The microsand with the floc attached settles to the bottom of the clarifier, where it is collected and recycled by a centrifugal pump to the injection hydrocyclone. The hydrocyclone separates the sand from the sludge, and the sand is re-injected into the process.

The mobile clarification unit

USFilter's mobile clarification trailers, incorporating the Actiflo process, can clarify up to 1,000 gallons per minute (gpm) of rapidly fluctuating raw water sources. Using this kind of system can help guarantee consistent water quality, ease of operation and a small footprint - making it suitable for facilities with limited space, and in applications requiring frequent start-ups and shutdowns.

People often require mobile clarification for emergency services, and to fulfil a need for supplemental water treatment. A mobile clarification system can be quickly delivered to municipalities and to customers in industries such as hydrocarbon processing, chemical processing, food and beverage and power.

Mobile clarification works particularly well in short-term situations where an outage is planned, when a system fails, or when additional clarification is required due to either a change in water quality or an increase in the water demand for the application - as the case studies opposite show. In some cases, the mobile clarifier systems can even provide a longer-term solution to the customer's need.

As well as the applications looked at in this article, mobile clarifiers can also be used for dewatering ponds, containments and collected groundwater in construction sites to prepare it for discharge. They can also be used to prepare surface water for further treatment, to provide drinking water when primary water source is unavailable, as well as for large industrial complexes that take in surface water and clarify it for use throughout their facilities. These include refineries, chemical manufacturers, steel mills and paper mills.

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