By Stephen Hinton

The first ever DynaSand unit was installed at Gunnebo Industries, in Västervik, Sweden, and has now been in continuous operation for 25 years. DynaSand’s inventor, Hans F Larsson, and Gunnebo’s previous operations manager, Bertil Wiberg, visited this site to share experiences and insights. In the DynaSand filter fouled sand is continuously removed from the filter bed, washed and recycled without interruption to the filtration process.

A hundred years of production
Gunnebo Industries’ installation is situated in a breathtakingly beautiful valley on the Swedish east coast. Still producing today, the facilities date back to 1764 when the factory was set up to produce nails for the nearby shipyard. This perfect setting affords a natural harbour for supplying the coal for the furnaces and water power to drive the hammers to form the nails.

However, hundreds of years of production leave their mark on the surrounding countryside. Nail production produces quantities of iron oxide formed during the cooling process, and zinc from galvanizing along with traces of chrome and lead.

Environmental awareness and trouble with the locals
During the 70s as environmental awareness was on the rise, along with environmental policing and legislation, the operations manager of the time, Bertil Wiberg was looking for a way to demonstrably curb factory emissions, and stay on good terms with local authorities and inhabitants.

Says Bertil; “ on the bottom of the estuary there is a layer of iron oxide from two centuries of nail, chain and wire production. It is covered by sediment and plants and is completely harmless. Earlier, emissions would muddy the waters which did not go down well with the local population. In fact, whenever anyone found usually high concentrations of metals in the area – or far out in the archipelago – suspicions would always be directed towards us”.

Bertil continues: “so I was looking to set up a plant to remove the metals from the waste process water and monitor effluent. We needed sand filters, but conventional backwashed sand filters were a problem as they needed a lot of space, especially as for continuous operation we would have to install two. The solutions would have been complex as well”.

DynaSand's first customer
I visited the company that is now Nordic Water in 1978 after hearing about the DynaSand prototype. I was immediately impressed by the simplicity of the design and its effectiveness and we decided to go ahead.

The DynaSand inventor, Hans F Larsson continues; “at the time all unit operations in chemical engineering and water treatment were continuous except sand filtration We had been working on the solution for several years, and were hopeful that this, deceptively simple device with no moving parts, had the potential to revolutionise the way we clean water”.

Gunnebo installed two filters, one for the galvanising line and the other for the pickling line. Still in operation today, the filters each comprise 1m2 filter area, with glass fibre reinforced plastic tank walls.

How DynaSand works
In the DynaSand filter fouled sand is continuously removed from the filter bed, washed and recycled without interruption to the filtration process. The DynaSand filter is based on the counterflow principle. The water to be treated is admitted through the inlet distributor (1) in the lower section of the unit and is cleaned as it flows upward through the sand bed, prior to discharge through the filtrate outlet (2) at the top. The sand containing the entrapped impurities is conveyed from the tapered bottom section of the unit (3), by means of an air-lift pump (4), to the sand washer (5) at the top. Cleaning of the sand commences in the pump itself, in which particles of dirt are separated from the sand grains by the turbulent mixing action. The contaminated sand spills from the pump outlet into the washer labyrinth (6), in which it is washed by a small flow of clean water. The impurities are discharged through the wash water outlet (7), while the grains of clean sand (which are heavier) are retained to the sand bed (8). As a result, the bed is in constant downward motion through the unit. Thus, water purification and sand washing both take place continuously, enabling the filter to remain in service without interruption.

The waste water treatment plant today
Today, the waste water treatment plant handles waste from the three different activities on site: chain production, electrolytic galvanising, nail production using hot galvanising and wire drawing.

Starting in a collection tank, water is feed through various tanks where flocculation is controlled by adjusting ph. Sludge is removed by a lamella separator, and the DynaSand filters provide the final cleaning step. Samples of effluent are collected continuously.

Emissions at the minimum
”The permitted limit of emissions is 2.0 mg/l. Our average is around 0.5 mg/l. In fact, we obtain our process water from the stream that runs through the factory. The water we put back into the stream is much cleaner than we take out”, says Lars Svensson, current operations manager.

”We also adjust ph values so emitted water is at ph 9.2. We do this in collaboration with the local authorities as a measure to combat acidification.”
“We now remove the mill scale on the wire mechanically”, says Lars Svensson. By running the wire through a device that causes it to bend slightly, the scale which is glass hard cracks off.

25 years of continuous operation
Standing in front of the DynaSand filters, former operations manager Bertil Wiberg and current operations manager Lars Svensson concur: “They have more than fulfilled expectations, performing well from the start and requiring almost no maintenance over 25 years”.

“The DynaSand filters have required very little maintenance. We have replaced the pumps twice, that’s all. The maximum wear on the inside surface of the tank is around less than 0.1 mm”, says Lennart Ekdahl, in charge of operation of the utilities.

“The secret to getting waste water processing to function effectively is to ensure a steady flow though the system, and to adapt waste handling as manufacturing processes change. Modern variable speed pumps allow us to control the process much better. The set up with the DynaSand works well for a wide range of effluent concentration, but no system handles overloading”, says Lars Svensson.

Reflecting back on some of the main lessons from the past 25 years Bertil Wiberg says “this is a case where subsidies have helped progress. They allowed us to build the waste water processing plant providing 25 years of clean discharge”.
Says inventor Hans F Larsson; “this installation demonstrates that the DynaSand filter really does provide the simple, effective continuous solution we set out to develop. There are now more than 15 000 of these filters operating worldwide in a wide variety of applications and even after 25 years I still think we have only just started.” SLDT