OC-1 Filter and Media in a Domestic Swimming Pool

Why I chose the OC-1 filter

Pool size 15 mtr X 4.5 mtr Pool shape Rectangular with Roman steps
Pool type Delifol vinyl liner UK Situation Inground Worcs UK
Capacity 50 cu mtrs Use Occasional seasonal domestic
Pump Certikin Swimflo HPS031M Flow rate at 8 mtr head 6.3 cu mtr hr
Filter Certikin ZZOC-1/20 Filter Diameter 20"


Once I had decided to go with OC-1 filter media I was presented with the choice of modifying my existing filter or buying a specialist made for OC-1 filter. Why did I chose to go to the expense of the specialist filter instead of modifying my existing 16" Lacron 406 side-mount filter? After all converting an existing filter for OC-1 is supposed to be fairly quick and easy with minimal cost. And it is, although perhaps not so quick and easy as claimed. I initially considered following the modification route, and went through most of the process, only to go in the end for the specialist filter.

Modifying an Existing Filter

Do the laterals have to be modified? According to ocmproducts, the filter will still work without modifying the laterals, but the modification will increase flow and lower resistance, and without modification 'the laterals could block, from the outside to the inside', which implies that there is debris passing right through the filter which is rather disconcerting. This means that particles greater than 0.5 mm (the low limit of the sand size) will pass right through the OC-1. But doesn't OC-1 trap 99% of 20 micron particles in one pass? Whatever happens, more frequant backwashing with unmodified laterals is advised to prevent this.

Modifying an existing sand filter requires two operations, amending the laterals and fitting a diffuser to the internal inlet pipe. One of these tasks is far easier than the other. I could find no instructions on ocmproducts.com except for commercial filters, which use a more extensive modification process (I do not think that domestic filters would need the shrink wrap on the laterals).

Modifying the laterals involves drilling a series of 8 mm holes in their undersides (Wouldn't it be great if Certikin sold modified laterals?) The commercial filter modifications have the only formula I could find for determining how many holes are needed, and here it is:

Filter Flow rate / 200 = Total Holes / Number of Laterals = Holes per Lateral

My 16" filter has an expected flow rate of 5,000 litres per hour, so 5,000 / 200 / 6 = 4.2, rounded up to 5 holes per lateral.

The flow rate is the expected flow rate in litres through the filter, not the maximum. One hole should be through the outer end of the lateral. The holes are positioned in the underside of the laterals - facing downwards.

Laterals seem to be made of some sort of polycarbonate, they have a little flexibility and are fairly tough. They are a closed tube with fine lateral slits which allow water through but not the 0.5 to 1.5 mm grains of filter sand. However attempting to drill a series of 8mm holes in fragile laterals is most likely to end in tears as the slotted laterals splinter. It's far better to heat up the drill shank and melt the holes, gently twisting the drill and easing it in and out as the hole is formed. This creates a moulded rim around the hole which holds the slotted edges together.

It seems to me that the lateral modification for domestic pools is too destructive. The laterals are smaller than in commercial filters, and the act of pushing several 8 mm holes in them is fraught, and an oversized hole could let the OC-1 media into the pool, which would not be a good thing. Perhaps it is better use a heated 7 mm or even 6 mm drill, and add an extra hole to compensate.

None of this information, nor the flow rates detailed below, or the quantities required, is reproduced anywhere on the bags of OC-1, just where it should be.

Filter Flow Rates

The flow rate for OC-1 media can range from 10 to 60 m³/m²/hr, with a recommended rate for optimum filtering at 25 m³/m²/hr. Unlike an absolute flow rate, such as from a pump, the filter flow rate is specified in cubic metres per square metre an hour. The 20" OC-1 filter has a filter area of 0.18 square metres, making the flow through the filter 25 x 0.18, 4.5 m³/hr, easily achievable and just as easily exceeded. A flow rate chart from Certikin confirms this.

Both sand filters and OC-1 filters are pressure filters, in that water is pumped though the media under pressure. In general a slower flow rate gives better filtering (especially for sand filters). To digress a little, OC-1 has a recommended flow rate and can operate at quite high flow rates. For sand, however, the slower the flow rate the better. In a sand filter there is a conflict between the more effective slow filter rate and the need for a higher rate for backwashing etc. With OC-1 the filter rate and the backwash rate are more closely aligned.

After modifying the laterals in my filter I decided not to use it. It was really too small for the pool, the multiport valve needed replacement and I couldn't get the components to mount the new valve, and it was forty years old, even though it was still quite servicable. I decided to purchase the smallest OC-1 filter Certikin sold, a 20" side mount OC-1 filter ZZOC-1/20.

The ZZOC-1/20 Filter

Whilst many outlets sell the OC-1 media, very few seem to offer the specialist filters - I could omly find around four in the UK. The OC-1 filter has more complex internals than a sand filter, it includes the OC-1 media, and is surprisingly competetively priced against a comparable bobbin-wound sand filter. During my searches I also found OC-1 filters offerred by AstralPool, but these seemed expensive and only available on the continent. AstralPool appear to be part of the Fluidra group, as are Certikin.

The OC-1 filter is similar to the well-known sand filter, it's hard to see any difference from the outside. On the inside however it has both lower laterals and a set of top laterals. The top laterals diffuse the water over the entire surface area of the media, and slow down the incoming water flowing through the media: they also prevent any media being sucked into the pipework on backwash. The OC-1 filter can't be used with any media other than OC-1, sand would be a disaster.

OC-1 Filter Internals

The laterals are different from sand filter laterals. in that they have larger rectangular slots which, with the open cell of the media, have far less resistance to the water flow. Although they do not have the array of fine slits as found in sand filter laterals, they have two rows of slits diametrically opposed to the rectangular slots. I can only guess what these slits are meant to do. There is no slot in the end of the laterals. The rectanglar slots in the top laterals face upwards, and in the lower laterls face downwards. Both sets of slots are positioned slightly off the perpendicular, whather by accident or design I don't know. There are ten slots in each lateral, and eight laterals top and bottom.

OC-1 Filter Top Laterals, slots slightly off perpendicular

OC-1 Filter Bottom Laterals

The two rows of fine slits can be seen in the bottom laterals picture

I had expected the filter to be plastered with warnings that only OC-1 should be used, and do not fill with sand, but nothing. There is a small label positioned halfway down at the rear of the filter, which gives the filter area and the maximum flow rate, but nothing else. There is no OC-1 quantity or fill line or anything. The brochure with the filter was equally as lacking in information. The filter was supplied with 70 litres of OC-1. If the height of the filter is say 0.5 mtr, and the area 0.188 sq mtr, then the volume at 2/3 capacity is just over 60 litres. In the end I used just about 69 litres, filling the filter to the top of the upper inlet pipe. I shall probably keep the spare OC-1 pieces to give away to the curious.

OC-1 Filter Label

The filter area is, at 0.18 m², nearly 40% more than the old 16" Lacron filter. With this increased area, and the specific made-for-OC-1 laterals, I hope to get a marked improvement over the previous setup.


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