Keeping standards high
Following a series of premature failures of sodium hypochlorite tanks due to the wrong materials being specified, the British Plastics Federation expresses concerns on the storage of sodium hypochlorite solutions in treatment plants
With the high levels of available chlorine, up to 15%, in a fresh solution, many thousands of tonnes are consumed on a regular basis. For drinking water treatment, the available chlorine content is ideal for dosing. Where odour control on WwTWs is controlled by scrubbing, sodium hypochlorite is commonly used as a scrubbing media.
Rubber- or ebonite-lined carbon steel tanks were used for bulk storage tanks until the introduction of the plastics materials in the late 1950s, namely pressed unplasticised polyvinyl chloride sheet (UPVC) reinforced with glassfibre reinforced polyester resin (GRP).
During the 1960s, the low cost and ease of manufacture of other thermoplastic materials led to materials such as polyethylene being used. They still are. Despite the lower cost of thermoplastics, the recognised materials of construction for long-term storage of hypochlorite solutions remains UPVC reinforced with GRP as it is proven by more than 20 years of case histories.
During the 1960s, European interests followed the low-cost all-thermoplastic tank developments and medium and high-density polyethylene (HDPE) was specified for storage duty. Following a number of failures caused by environmental stress cracking, the most widely used and detailed tank design standard, the German Merkblatt DVS 2205, omitted to list sodium hypochlorite within the Standard as a suitable application for tanks in polypropylene and HDPE. The relatively new and far less stringent European Standard BS EN 12573 - 2000 also omits sodium hypochlorite in the listings for the chemical factor.
In the UK and other parts of the world, this advice from the German manufacturers was ignored, and many thousands of tanks were produced in polyethylenes of various types and methods of manufacture. It must be said at this stage that some of these tanks fared better than others.
The length of service, before leakage, has varied from three years to more than nine years. Having looked at specific applications, some tanks were very infrequently filled, thus the active available chlorine levels were considerably reduced due to the relatively unstable nature of hypochlorite solution.
False impression of resistance
This may have given a false impression of the resistance to the concentrated fresh solutions. This is further confirmed by the relatively short life of thermoplastic tanks experienced where there is high consumption of fresh solutions on a regular fill cycle.
From the 1970s through to the 1990s, polyethylene was specified and used, with some manufacturers giving guarantees of up to ten years. In many cases, these guarantees were not followed up, as a number of tanks suffered leakage before this length of service was achieved, particularly in high-usage facilities where fresh hypochlorite solutions were always present.
While some end users still specify HDPE for storage of sodium hypochlorite, neither the long-standing German standard nor the pan-European standard list HDPE for the storage of 15% sodium hypochlorite. Concern is expressed by the polymer manufacturers such as Basell and by the German national standards authority, TUV.
The technical manager of the world's leading polymer manufacturer, Basell, has recently said: "Parts made of Hostalen PE grades (the most chemically resistant grades) can be applied in contact with Sodium Hypochlorite at 30°C for one year and at 20°C for four years. After this time period, we recommend exchanging the containers."
With the ever increasing regulations designed to protect the environment and people's safety, the majority of manufacturers of chemical storage tanks in the UK no longer consider HDPE as a suitable material. Tanks manufactured in HDPE may be considered for short-term use as demonstrated by the statement above. But, long-term, there is a very high liability placed not only on the manufacturer but also the specifier and the end user.
Most WTW installations have a design service life of 20 years or more. From in-service experience, this is not being achieved with tanks constructed or moulded in polyethylene. An alternative material of construction to UPVC / GRP, which gives the required service life, is a vinylester laminate. During the 1960s, the US developed vinylester resin systems, which proved to be highly resistant to chlorine environments such as 15% available chlorine hypochlorite solutions.
The only safe options
Following the material manufacturers' recommendations and the experiences in the field, it is evident that for long-term storage of high concentrations of sodium hypochlorite, either UPVC reinforced with GRP or a full post-cured vinylester GRP laminate are the only safe options to be considered. Such tanks must be designed and manufactured to the very extensive British Standard 4994 -1987. Unfortunately many specifiers are persuaded to opt for the much lower cost option of an all-polyethylene thermoplastic tank, often quite innocently, as the manufacturer states an expected service life in excess of ten years. In many instances, these guarantees are not implemented due to being lost in the mists of time.
Many WTWs now incorporate chlorine injection into the water supply using on-site generated sodium hypochlorite. This solution is produced on a continuous basis from a brine solution, which is passed through an electrocatalytic cell, producing a low-strength solution with a low pH.
These solutions are often quoted as 0.8%-1% strength sodium hypochlorite. Due to the low pH (about pH 9 - 10) conventional hypochlorite is stabilised with caustic soda with a minimum pH 13), the chlorine content is highly aggressive. Traditionally, chemical grade pressed UPVC-reinforced with GRP has given good service. This highly aggressive material can accelerate the problems of stress cracking seen in polyethylenes. Even the vinylesters successfully used for the conventional hypochlorite storage tank can be attacked if a specially formulated system is not incorporated and fully post cured in an oven at 90 degrees Centigrade for ten hours.
Caution must be exercised when specifying bulk storage tanks for sodium hypochlorite solutions. The liabilities generated by potential leakage or catastrophic failure are now so onerous that engineers and end users must be confident they are receiving the correct advice. Whilst cost considerations must play a part in the selection of a sodium hypochlorite storage tank, medium- to long-term safety must be paramount in the mind of the specifier, end user and the manufacturer. The correct materials of construction, as outlined above, together with the design and manufacture strictly conforming to British Standard 4994 - 1987 must be specified on the grounds of safety and lifetime costs.