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Pool water hygiene - dealing with the consequences of lockdown


Whilst industry bodies and leisure operators are busy planning how social distancing can be maintained when commercial pools reopen, there appears to be less awareness of the potential water hygiene problems created by facility closures.

Andrew Gaffey, Technical Director, Gaffey Technical Services Ltd.

At the time of writing, 10 weeks into the UK social distancing restrictions, we've learned of algae and biofilm problems occurring at a number of commercial and public pools across the country, and it seems a pattern is beginning to emerge. A fairly common practice by pool operators appears to have been to reduce the pool turnover rate to save energy during the lockdown period, however, it is becoming obvious that this practice could have a significant flaw.

Whilst many pool circulating pumps are equipped with variable speed drives, reducing the turnover rate of the pool can completely change the hydraulic distribution pattern of the water volume. Pools are designed to operate on a specific turnover period for good reason; inlet velocity is key to making sure freshly chlorinated water can be propelled into the far corners and depths of the pool. Without sufficient velocity "dead spots" can occur. Areas where chlorine cannot be replenished become a perfect habitat for algae and biofilm growth to become established, especially if the pool is exposed to daylight.

A simple two-stage dye test can reveal how effectively your pool water and chlorine distribution is performing.

A better approach?

Rather than running the pool continuously at a low turnover rate, instead, run at normal turnover speed during daylight hours, at least. If you must, switch to a lower turnover rate at night. This way, pools which are exposed to daylight (which both depletes chlorine and encourages algae growth) will have good water movement and ample chlorine distribution during daylight hours. If a UV treatment system is installed there will be no real benefit in operating it whilst there are no bathers present; chloramines will not be generated at any appreciable level and the UV treatment effect will simply add unnecessary chlorine demand, potentially more phosphate (algae fertilizer) and additional pH correction demand.

How to prevent and treat algae

Algae thrives on phosphate and daylight. Phosphate can be found to a varying degree in the water supply to prevent lead-leaching in the supply network, but moreso in common pool chlorination chemicals to reduce scale formation) and general cleaning products that are often used to clean pool decks and changing areas. Many general cleaning products contain surfactants which can have a very detrimental effect on flocculation and filtration too, so these products need to be kept well away from the pool water. Less hazardous 'organic' based cleaners, such as citric acid, react with chlorine to produce high levels of chloramines and THMs, so whilst these products may be environmentally friendly they are usually not poolwater friendly. Inorganic chemicals should be used instead; sodium bicarbonate for removing greasy deposits on scum lines or pool decks, or weak hydrochloric acid 5-10% for scale removal, whilst wearing suitable PPE of course. So, to control algae it's important to minimise the amount of phosphate entering the pool as far as possible.

Commercial chlorine chemicals such as sodium and calcium hypochlorite often contain phosphate additives which are used as anti-scale agents, so it might be worth checking which type of chlorine product you are using and whether you really need the anti-scale versions. On-site generated hypochlorite does not contain phosphate or additives and doesn't cause any scaling of injection points. It's a disinfection method which is growing ever more popular in commercial pools and is worth bearing in mind, particularly if your pool is prone to algae problems.

Daylight exposure can be minimised using UV resistant pool covers or by covering windows whilst the pool is closed, but care needs to be taken with pool covers as they can also become a breeding ground for biofilm (see below). For closed pool facilities which are already suffering from algae, the key remedial steps are:

  1. With the pool operating at normal turnover rates, super-chlorinate the pool to at least 10mg/l free chlorine and ensure the pH is corrected to ideally between pH7.0-7.5. To calculate the chlorine dose, for every 100 cubic metres (22,000 gallons) of pool volume you will need to dose 6 litres of 14% sodium hypochlorite, or 1.4kg of 70% calcium hypochlorite.
  2. Physically loosen algae by manually brushing the walls and floors of the pool. A pool cleaning robot is unlikely to be powerful to remove stubborn algae. Allow the pool circulation to continue for 24-48 hours, with occasional brushing to re-suspend algae debris in the water, and checking that an ample residual of free chlorine remains throughout this process.
  3. Thoroughly backwash the filters and top-up the pool.
  4. Add an initial dose of a proprietary algicide to the pool and then continue with a weekly dose, best applied after any filter backwashing. Different strengths of copper and copper-free algicide are available and the choice is likely to depend on your source water chemistry and pool fabric, so get some advice from a knowledgeable pool chemical supplier before buying.

It should be noted that algicides are intended to prevent algae rather than cure it. Superchlorinating the water and periodic physical brushing will have the best effect when initially trying to kill-off algae.

How to prevent and treat biofilms

In simple terms, biofilm is a slimy organic substance often appearing semi-transparent or grey in colour, which forms when bacteria begin to colonise on surfaces and secrete lipids, proteins and polysaccharides. Biofilm is a very different problem to algae in that it can proliferate in darkness and thrive in chlorinated water. Pools with inefficient filtration and high turbidity are especially prone to biofilms. Fine particulate organic matter can accumulate on pool and balance tank surfaces, creating a nutritious substrate to support bacteria growth. As the biofilm grows it serves as a protective barrier, making the bacteria within difficult or impossible to eradicate with traditional pool chlorination or superchlorination treatment.

In spa pools and warm water pipe circuits biofilm can easily become a breeding ground for Legionella. After a long period of facility closure it is vital these systems are thoroughly and professionally cleaned before they are put into use, even if they have been emptied during closure.

Legionella is not a concern in ordinary swimming pools where aerosols are not generated, however, biofilm is well known for harbouring Pseudomonas Aeruginosa and other bacteria which can be harmful to the health of bathers.

Surfaces of balance tanks, pool overflow channels and pool surfaces are some of the most common places to find biofilms.

Pool covers are also prone to biofilm contamination, as they sit on the surface of the pool where greasy particles and pollution are typically found in higher concentrations, particularly in pools that are not of deck-level design. When pool covers are removed they are usually stored on a pool-side reel, or rolled-up and stored in warm and humid pool-side store rooms; either way, a great environment for bacteria growth. For this reason, pool covers should be periodically disinfected and physically scrubbed, which can be a sizeable task!

An simple test for pool cover contamination: – Once the pool has closed and the chlorine dosing has achieved setpoint (and any UV treatment is switched off), measure the total chlorine usage of the pool overnight whilst the cover is in place. You could simply mark the level on your chemical dosing tank, or, if using a dry chemical feeder, empty it and refill with a known weight of product. The next day, repeat the test without the pool cover fitted. If there's a significant increase in chlorine consumption when the cover is in place then it's a good sign that contamination is present.

In swimming pools, a biofilm problem often reveals itself by way of an unexplained and sustained increase in chlorine demand, or sometimes by a positive Pseudomonas Aeruginosa test, which often returns again within days of super-chlorinating the pool. More chlorine will not solve a biofilm problem.

The crystal clear benefits of installing a good quality flocculation system

Good quality filtration plant and a correctly installed and calibrated flocculation system can be very beneficial in reducing the background organic load in the pool water, reduce chlorine demand, and the pool's susceptibility to biofilm growth. That's all good to know, but if you are a few weeks away from re-opening your pool you may not have time to fix these issues right now!

Effective treatment and prevention of biofilm

Chlorine dioxide (ClO2) is well known and proven in the water treatment industry to be highly effective in destroying and preventing biofilms. In treating spa pools, a strong and sustained residual of ClO2 for several hours can be very effective at clearing biofilms from pipe-lines and filters, before neutralising, emptying and refilling the spa. Using this method, the LegioCid® Plus treatment kit is a highly effective solution to chemically 'scrub' and disinfect the entire circulation and filtration circuit and all wetted surfaces - something which is virtually impossible to achieve by manual cleaning. In swimming pools this type of treatment is not practical, so the chlorine dioxide dose needs to be much lower and exist for an extended period of time to effectively purge biofilms. The use of chlorine dioxide solutions or tablets is not recommended for the long term treatment of recirculating systems such as swimming pools, due to the potential for chlorate build-up in the pool water.

One excellent solution to this problem was developed in the late 1980's by an Austrian company, Wapotec GmbH. Wapotec developed and patented a TCDO (tetrachlorodecaoxygen) compound, Hydroxan®, a non-toxic and non-hazardous precursor/oxidizer, which provides a slow and controlled release of chlorine dioxide without the build-up of chlorate by-product. Hydroxan can be dosed into a pool continuously in very small quantities (typically less than 1 litre per day for a 25m pool) where, in the presence of normal pool chlorine levels, it is activated by an increase in organic load, provided by bathers entering the pool. Producing a highly effective oxidation boost throughout the whole of the pool's hydraulic circuit, the chlorine dioxide residual created by Hydroxan (typically 0.2ppm) helps to significantly reduce the free chlorine demand of the pool whilst providing robust protection against biofilm formation and associated pathogenic bacteria such as Pseudomonas Aeruginosa. Chlorine dioxide is a much more selective oxidiser than chlorine and does not react with ammonia nitrogen compounds, therefore avoiding the formation of chloramines or nitrogen trichloride. With a very small but continuous dose of Hydroxan, a low chlorine dioxide residual is maintained throughout the whole pool and hydraulic circuit, preventing biofilm growth even in the hardest to reach parts of the pool circulation and filtration system – and your pool cover.

Other TCDO products such as Desopur-F® are available for the treatment of pool inflatables, foam swimming aids and all wetted surfaces where harsh chemicals are not compatible with contact surfaces.

Gearing-up for reopening

From the anecdotal reports received so far, it is likely there will be hundreds of pools in the UK affected to some degree by biofilm or algae contamination, and which will need to be treated and tested before they can be considered hygienically safe to reopen. From past experience it can take from several days to two weeks to achieve remediation of a pool, plus some additional time for lab test results to be returned. Many facilities may well need the help of an experienced pool service company and the demand on them will be great, as they will also be dealing with lots of other equipment issues too. Now is a good time to have your pool checked out, get tooled-up, and avoid any nasty last-minute surprises!

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