OC-1 Filter Media in a Domestic Swimming Pool

How is OC-1 performing so far?


Pool size 15 mtr X 4.5 mtr Pool shape Rectangular with Roman steps
Pool type Delifol vinyl liner UK Situation Inground
Capacity 50 cu mtrs Use Occasional seasonal domestic
Pump Certikin Swimflo HPS031M Flow rate at 10 mtr head 8 cu mtr hr
Filter Certikin ZZOC-1/20 Filter Diameter 20"


Although we have used the pool on and off since the late 1980's, managing the pool has sometimes been hit and miss with perplexing results. After progressing from throw a puck into the skimmer once a week towards the purist attitude espoused by troublefreepool.com, I believe that we are now more aware of what we are doing. But that doesn't stop neglect, and for various reasons the pool has been ignored for over two years, in the hope that it would perhaps go away. Indeed the idea of filling it in was considered. As for a cost and time evaluation against actual use, a pool is probably not a good investment, rather like growing your own vegetables. A lot of work and a few moments of priceless pleasure.

It must be emphasised just how bad the pool was at the start of 2020. The pool has had an Abrisud low level cover acquired second-hand and fitted around 2010. It is absolutely child-proof, keeps large debris out, warms the water like a huge solar panel, and is lovely to swim under in the rain and at night. However it's pretty immovable for us, and underneath it's warm and moist and most importantly out of sight. After two years of wilful neglect in spring 2020 the end cover was moved back to reveal

The pool at opening April 2020

The look of the water is deceptive - it is the milky turquoise colour found when a green pool is chlorine shocked. Although there was no pump running or filtration, the pool was shocked by adding liquid chlorine (14% sodium hypochlorite) and stirring the water with a brush. Visibility was around 50 cm, or 18 inches. The external vegetation was easily cleared, and then a deep leaf net used to start pulling all the rotten, stinking slimy debris from the bottom of the pool. (The pool sides are sloping, as is the floor down to around 7 feet in depth.) There was a huge amount, all cleared blind as the water was like pea soup. The pool consumed gallons of chlorine and kilograms of pH-. Meanwhile the pump and filter pipework continued, until:


The pool water level was topped up, and after a delay from a tremendous thunderstorm in the afternoon, at 17.00 on Tuesday 16th June 2020, at least a month behind the plan, the multiport valve was set to waste, the pump was primed as best I could, and turned on. The initial priming took a long tme, worryingly so, but eventually full flow was achieved. After a few minutes to waste, the valve was switched to filter and the air valve on the filter opened. The filter filled, and settled down to a low pressure of just under 0.5 bar, or 6 or 7 psi, rising slightly during the evening. The normal pressure at the start of a filtering cycle is, according to the OC-1 filter brochure, 0.8 kg/cm² or 11.4 psi (but the gauge is in bar, which is as near as dammit to kg/cm²). The OC-1, or some of it, rose up to the filter lid and stayed there, these particles will drop, I trust, over the coming days.

Right from the start the first objective was achieved. No filter media is ever going to get into the pool.

Filter Pressure at startup 16/06/20

Brush head at startup just visible in 750 mm water 16/06/20

The pool water was not unattractive, mostly blue but rather murky, presumably due to both dead and live algae and suspended solids. It was possible to make out the head of the brush in the shallow end (750 mm) but only just. The pump was left running continuously with both low suction and the skimmer open, and at the same time the work to clear the debris from the deep end continued. By the last day of May the water was warm enough for me to be in the pool and brushing. Much of the pool was unpleasantly slimy underfoot.

Over the next few days the pressure held at 7 psi, and the water was still cloudy. The OC-1 fell back from the filter lid, and the top laterals could be seen working. The flow seemed quite feeble, but there are a lot of slots in the laterals, 80 in total, to take the flow.

Whilst waiting for the system to settle in, what exactly is a flow of 5 m³/hr? ABS 1.5" pipe has an internal diameter of 42 mm whch gives an area of 1,385 mm². One metre of pipework therefore holds 1,385,000 mm³, which sounds a lot but is actually 1.385 litres. Coincidentally 1.385 litres a second is 5 m³/hr, so the desired flow through the pipework is one metre per second. After all that I still don't know if I am getting 1.385 litres a second, the return to the pool is very strong and almost certainly exceeds this.

I had expected the flow in the filter to be strong, but it isn't. However at a high flow rate, say 12 m³/hr, the flow is 200 litres a minute, 3.33 litres a second. There are 80 slots in the laterals, so even at a high flow rate each slot emits 42 millilitres a second, which isn't much, not even a mouthful (and there are the two long slits in each lateral to further reduce the flow out of the slots). At 5 m³/hr the water gently burbles out of the slots.

Early July 2020:

Everything grinds to a halt. The pool water is still cloudy. The pump is pulling 650 watts, is blisteringly hot, and using about £20 of electricity a week. It is a 0.5 HP pump, not a 0.3 HP. It is also making some funny noises. After a great deal of effort I finally hit upon a comment in a USA forum that the power demand of a centrifugal pump is proportional to the head times the flow rate. The flow at the return certainly seems very powerful, but I don't have a flow meter. I have found another reference that says that when the pump is at the low head/high flow rate end of the flow/head chart it is at its highest power consumption. Now I know the centrifugal pump affinity laws. I made a lot of effort to reduce the friction in the system and now the pump is suicidal. I think that the high flow rate, which may well be around 60 m³/m²/hr, is too fast for the OC-1 to cope.

Certikin Tech devision have been helpful in this, but can't explain the pump labelling. They have contacted the manufacturer in Spain for an explanation. The pump suppliers have checked their stock and other HPS031 0.3 HP pumps are labelled the same (as 0.5 HP).

July 2020:

At long last I fitted a ball valve in the return pipework fairly close to the return from the MPV. The ball valve was partly closed to achieve a filter pressure of 12 psi (the filter brochure recommends 11.4 psi). The pump altered its tone a little, and the flow rate in the return to the pool was greatly reduced from a raging torrent to a gentle burble. I can only guess at flow rates, probably up to 10-12 m³/hr before and now 3-5 m³/hr (the target is 4.5 m³/hr). Unfortunately the watts used didn't fall as far as I expected, only by 70.

The water is gradually clearing: whether it will clear faster with the flow rate closer to the recommendation for OC-1 we shall see.

Data after throttling return pipe

Summer 2020:

We've been swimming, it feels good, and the water is certainly clearer, but the deep end (7 feet plus) may as well be the Mariana Trench. The chlorine and pH- demand is still high, and the dredging process continues. The pump remains in 24 hr use, as noisy as ever. Things look better at the shallow end where the liner join lines can be seen, just.

Brush head visible in 750 mm water at the end of the 2020 season 07/11/20. Compare with the photograph above

The end of 2020:

The pool is closed for 2020, which in reality means I stop bothering with it, and the pump turned off in October. No additional tweaks, such as 'wintering chemicals', were used. After a lot of work during the year and quite a few tribulations, some noticable improvement can be seen. I think that the filter media has faced, as any filter media would face with this pool, a Herculean, or more accurately Sisyphean, task. Perhaps I was expecting or hoping for too much, and perhaps it would be too optimistic to rate the year as a success. I think the best description, after comparing the two photographs of the brush head in 750mm of water, taken at the start and end of the season, is Definite Progress.


The start of 2021:

Surprisingly the water was clear when the pool was opened. It had been given an occasional dose of chlorine and a stir over winter, nothing else. A moderate chlorine shock was applied, the pump restarted and filtering continued its 24-hour task.

Inside the pool the leaf net has extracted just about all the debris from the deep end, but the steps, most of the walls and half the floor are covered in a black slimy gunge. Brushing, with the old brush and a new one which quickly shed its bristles, had no effect except to cause me despair. I then used a new vacuum brush head on a pole: this brush has rather stiffer no-nonsense nylon bristles. To my amazement and heartfelt relief the black gunge simply peeled away in strips to be collected by the stalwart leaf net, leaving an almost pristine liner surface. This is a task which gave great satisfaction. The gunge was piled into two wheelbarrow loads and dumped on the vegetable plot. We shall end up eating it, eventually and indirectly.

Black algae peeled from the walls and floor 03/06/21

After the black algae had mostly gone the chlorine demand fell to around two litres of 14% a week, a loss of under 1 ppm per day, and the pH remained stable at around 7.4. There is still some debris to remove, some more difficult spots to brush, and some scale deposits to clear. As the dunked brush seems to have become the de facto means of comparison here is the 2021 season start photograph.

Brush head in clearer water at the start of the 2021 season 16/05/21

After a very cold May we have been swimming in June and the water has - for one day - touched 28 deg C. Although I have dosed the pool with Sea-Klear clarifier a couple of times I am not sure what effect it has had, so I am holding off adding anything else except chlorine and pH adjuster. I hope to go from hazy to clear to sparkling. At the moment it's between hazy and clear.

I have to say that what's been achieved in a year could have been done in far less time. If I had removed the cover and spent a week brushing and collecting the debris and then holding the water at chlorine shock level then I, and the pool, would be in a better position. But I didn't. The moral is don't pussyfoot around.

June 2021:

The water clarity has improved immensely, the chlorine loss is stable at under 1 ppm a day, and the pH is remains between 7.2 and 7.4. Now, at the start of June, it's up to, or down to, OC-1.

On a sunny day, at the shallow end, the water can look very nice indeed. But the photograph flatters to deceive, and deeper water gets hazier. The legs do not belong to me in any respect.

In rare sunshine 14/06/21

July 2021:

The water is still hazy. It appears to be clearing slowly, the floor at the deep end is quite visible, and we would be swimming if it weren't so blooming cold. But where is the one-micron clarity? I added 150 ml of Sea-Klear chitosan clarifier but that didn't seem to make a significat difference. I have had two contacts through these web pages saying that they too had hazy water, and one resolved this in days with a cationic clarifier. I also came across a pool supplier's website, where OC-1 was generally praised quite highly, with this caveat:

'There have been rare instances of water quality problems post-installation. All problems experienced that we are aware of have been on existing pools being converted from other filter media. The problems were found not to be the fault of the OC-1, but either poor installation or existing problems with the hydraulics or treatment/other management of the pool water that were masked by the entrapment method of filtration.'

Tantalisingly that is all. What were the problems and how were they resolved, and why were they specific to OC-1? Is there some aspect of filter management peculiar to OC-1 that passes unnoticed with sand as the media?

I ran the first backwash of 2021 to waste. The water appeared to be clear, and the contents of the filter were not as agitated as I expected, or hoped. There is no change in the filter pressure.

There seem to be a few options left, in this order. Either:

a) Carry on as now, hoping the water will eventually clear. But how long is eventually?
b) Carry on, but add regular Sea-Klear clarifier, in the hope that it is actually doing something
c) Try a one-micron sock on the return, such as a Slime Bag, if only I could find one for sale in the UK
d) Give in, and try a clarifier such as Swimfresh which is produced for OC-1 by Certikin, even though it appears to be a standard cationic clarifier

At the moment I'm at A, but I might weaken later in desperation.

There is an OC-1 fault finding and fixing chart, which mentions flocculation. Whatever happens, a flocculant is one of the last things I would use, with or without OC-1. All that vacuuming to waste. Let's hope they mean a clarifier, as clarifiers are not specifically mentioned elsewhere on the chart.

OC-1 Trouble shooting guide

The Flow Meter:

Now for the flow meter. I was not really struck on buying and piping a flowmeter at upwards of £100, so other means are required. I connected a short length of flexible pipe to a threaded hosetail, screwed it into the pool return, and measured the time it took to fill the bucket to the brim. The bucket's capacity was measured prior to the exercise at 11 litres. The tests were run twice with remarkable consistency. (It is better for another person to do the timing, whilst the instigator concentrates on getting soaked.) The results may or may not be as accurate as a proper flow meter, but they are not intended to be. All I need is to be in the right area.

Flow meter components 13/07/21

The unrestricted flow rate was less than I thought, at around 8+ m³/hr allowing for leaks etc. The filter pressure was at 6 psi. This is about 1/3 more than the high speed filter rate recommended for OC-1 in a 20" filter. Throttling the flow back with the ball valve on the return reduced the flow to 5+ m³/hr, just about spot on the optimum filter rate, raising the filter psi to 10. By keeping the filter pressure at 10 psi the flow rate can be held at the OC-1 recommendation.

Flow rate chart for open and throttled return 13/07/21

Mid July 2021:

The sun is shining, the pool temperature is hitting 30 deg C, we're swimming every day, and the water is undoubtedly looking very good indeed. It is now, at long last, justifiably well into the clear category. I would even say that my old cheap camera doesn't do it justice. The pool has now become an obsession, I am aiming for absolute, eye-splintering clarity.

From hazy to clear 21/07/21

How clear, measurably, is the pool water? I don't know as I don't have any instrument to measure it, but Severn Trent - our public water suppliers - do. They measure the turbidity regularly at the point of use and publish the results, which means that the pool water can be compared with the known and accurate turbidity of the public supply water. (Turbidity is expressed in NTU's: an NTU is a nephelometric turbidity unit, with a general accepted upper limit of 0.5 in public swimming pools.) The turbidity for the public supply in our zone is:

Severn Trent's public water supply turbidity measurements (NTU) 30/03/21

On a rather overcast day a sample glass of pool water was compared side by side with a sample of the public supply (although we have spring water we have a public water supply also). The comparison is of course made using eyesight and perception. The photograph can be opened full size in another tab for a closer look. I have to say it isn't immediately obvious which one is which, or to be more precise, which one is more turbid.

Pool water compared with public supply 10/08/21

The white flecks are on the green plastic lounger, by the way, not in the water. As - to my fading eyes - there's hardly any difference between them I am going to claim that the pool water turbidity is comparable to the public supply at somewhere around 0.18 NTU. If you're still not sure which glass holds which water, the answer can be found by hovering here. Actually I think the pool water is very slightly clearer, but then I would, wouldn't I?


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