Control of the Biofouling Bryozoan, Plumatella Repens, Using Pulsed Chlorine Treatment
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Int. J. Environ. Sci. Technol. (2013) 10:199–208 DOI 10.1007/s13762-012-0117-8 ORIGINAL PAPER Control of the biofouling bryozoan, Plumatella repens, using pulsed chlorine treatment R. C. Mant • G. D. Moggridge • D. C. Aldridge Received: 28 November 2011 / Revised: 9 June 2012 / Accepted: 2 August 2012 / Published online: 3 November 2012 Ó CEERS, IAU 2012 Abstract Bryozoans are common biofoulers of under- is suggested that upstream processes are likely to create drain filter nozzles in rapid gravity filters in water treatment sub-optimal conditions for bryozoan growth. works. A potential method for controlling bryozoan bio- fouling is the use of chlorine in backwash water. Repeat- Keywords Bryozoan Á Control Á Rapid gravity filters Á edly exposing bryozoan colonies with chlorine for 20 min Water treatment works every 24 h in an experimental setting, to replicate what would occur if the backwash was chlorinated, caused sig- nificant reduction in colony growth and size. After 10 days Introduction repeated treatment in good conditions for bryozoan prop- agation, the EC50 (the chlorine concentration required to Bryozoan biofouling decrease growth such that treated colonies were half the size of control colonies) was 1.6 ppm (SE 0.3). In sub- Bryozoans are suspension-feeding animals that form col- optimal conditions for propagation, the impact of chlorine onies attached to natural and artificial surfaces (Ryland was greater. The majority of colonies treated with 1 ppm 1970; McKinney and Jackson 1989; Wood and Okamura and above did not grow or even decreased in size over 2005). The basic units of the colonies are zooids, which, in 5 days. However, a chlorine concentration of 5 ppm was freshwater colonies, are connected by either a gelatinous necessary, even in sub-optimal conditions, to ensure all mass or chitin-coated tubes. Bryozoans, especially the colonies decreased in size over 5 days of treatment; this is tubular species, may be the most common freshwater too high to be acceptable to water companies due to the biofouler (Wood 2005). They cause a biofouling nuisance risk of carcinogenic by-products. Nevertheless, the in a number of settings including aquaculture (Dubost et al. observed decline in feeding activity of bryozoans exposed 1996), prawn farms (Baily-Brock and Hayward 1984), to chlorine levels [1 ppm suggests that repeated back- irrigation systems (Wood 2005), power plants (Aprosi washing with chlorine may cause colony death over time, 1998), waste water treatment works (Wood and Marsh especially in sub-optimal conditions. Chlorine backwashes 1999) and drinking water treatment works (Shrivastava and may therefore be an effective long-term control strategy, Rao 1985; Smith et al. 2004; Smith 2005; Smith and especially in locations such as rapid gravity filters where it Batson 2005). In drinking water treatment works in the UK they have been found growing on the filter nozzles at the bottom of rapid gravity filter beds (RGFs), creating back- & R. C. Mant ( ) Á G. D. Moggridge pressure problems, uneven scour during backwashes, and Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, have contributed to £1.49 million of refurbishment costs Cambridge CB2 3RA, UK within four water treatment companies (Mant et al. 2011). e-mail: [email protected] Bryozoans can be the only biofouler found growing on the RGF nozzles, but they are often accompanied by both R. C. Mant Á D. C. Aldridge Department of Zoology, University of Cambridge, hydroids (e.g., Cordylophora spp.) and sponges. Not all Downing Street, Cambridge CB2 3EJ, UK water treatment works (WTWs) with biofouling problems 123 200 Int. J. Environ. Sci. Technol. (2013) 10:199–208 have visible bryozoans on their filter nozzles, but their (0.3 mg/l) of chlorine, in the form of sodium hypochlorite, dormant dispersive phase, statoblasts, can be found at the if dosed over 24 h can keep bryozoans under control as a majority of works (Mant et al. 2011). In the UK WTWs maintenance measure but again no experimental data have these bryozoan statoblasts are most commonly from been presented. In terms of CIP, Smith et al. (2004) state Plumatella repens (Mant et al. 2011). that no colonies treated with CIP contained living polyps, but that there was well-preserved polypide tissue in many Potential control strategies of the samples, including the control; making it unclear as to the difference in death between the treatment and con- Bryozoans, as well as the other fouling organisms, need to trols and no data are presented. be controlled at WTWs to reduce biofouling of RGFs. A variety of control strategies have been tried against bry- The statoblast problem ozoans, but there is little reliable experimental data (Wood 2005). Control options for biofouling in general can be One of the most problematic aspects of bryozoan control is divided into physical and chemical controls (Jenner et al. the dormant resistant phase in their lifecycle. In the most 1998). common freshwater bryozoan, the Plumatellidae, this dor- In terms of physical control options, it has been reported mant phase is a statoblast (Wood and Okamura 2005). The that increased water velocity, UV, and sonication (Smith statoblasts are seed-like structures of about 0.4-mm length, et al. 2004) can reduce bryozoan biofouling, although none which are resistant to control measures (such as the use of stop growth completely and none could easily be applied chlorine), and many treatments that kill adult colonies will within an RGF. An alternative physical control strategy is not kill the statoblasts (Wood 2005). If the statoblasts are the production of anoxic or food-depleted conditions. not killed along with the adult colony, the colonies can Three hours of anoxic conditions usually causes mortality re-grow from these statoblasts (Wood 2005). Hence, the of bryozoan zooids (Wood 2005). Anoxic conditions, cre- control of the statoblasts is an important part of controlling ated by taking the filter beds off-line, are already used to bryozoan biofouling. control chironomids in granular activated carbon filter beds Of the chemicals tested, the germination of statoblasts is (Olsen et al. 2009) and could be used to control bryozoans reduced or stopped by chromic nitrate, ferric nitrate, in RGFs in the same way. However, neither anoxic con- chloroform, methyl alcohol, silver, mercuric and cupric ditions nor food depletion are likely to kill the dormant ions (Mukai 1977). None of these treatments are licensed statoblast stage of bryozoans. for use within WTWs due to their toxic effects to humans. The alternative to physical control is chemical control. It has been proposed that the higher level of resistance of Bryozoans are more sensitive to heavy metals (copper, the statoblasts may mean that the best method of control is zinc, cadmium and chromium) than many other inverte- to wait for them to germinate in the spring and target young brates and fish (Pardue and Wood 1980). However, heavy colonies (Wood 2005). metals could not be used within WTWs due to their toxicity As the statoblasts are hard to control, completely erad- to humans (WHO 2011). Chlorine (Shrivastava and Rao icating the bryozoans with a single dose is likely to require 1985; Wood and Marsh 1999) and a commercial cleaning overly high concentrations of any chemical toxin. There- solution Clean In Place (CIP, Veola Water Systems, Aus- fore, ‘‘maintenance’’ chemical controls using repeat dosing tralia, Smith et al. 2004) have been reported as potential at lower concentration are a more promising option. control solutions for adult colonies. CIP is sold in two forms: caustic and acid. Caustic CIP is of 0.25 % hydrogen Chlorine backwashing peroxide, 0.14 % EDTA sodium salt, 2 % sodium hydroxide, 0.2 % Memclean C and 97.41 % distilled water. A possible maintenance control strategy is the application Acid CIP is of 0.1 % sulphuric acid, 0.05 % EDTA sodium of chlorine treatments in the RGF backwash water, the salt and 99.85 % distilled water (Smith et al. 2004). water used to clean the RGF beds. Chlorine can be an However, experimental data on chlorine and both forms of effective control chemical at low concentration; 0.3 mg/l CIP is very limited. In terms of chlorine, both Shrivastava of chlorine, in the form of sodium hypochlorite dosed over and Rao (1985) and Wood and Marsh (1999) suggest that 24 h, has been reported to keep bryozoans under control as bryozoans can be prevented by chlorination of 5 mg/l. a maintenance measure (Wood 2005). Chlorine also has the However, both present no direct experimental data, additional benefit of reducing the growth of another RGF although Wood and Marsh (1999) state that bryozoans only biofouler, the hydroid Cordylophora spp. (Rajagopal et al. clogged the filter nozzles of sand filters in a water treatment 2002; Folino-Rorem and Indelicato 2005; Mant et al. works in WI, USA, when chlorine levels fell from 7 to 2012). If multiple organisms are biofouling a single sys- 2 mg/l. Wood (2005) also reported that a much lower dose tem, it is more cost effective to control all of the organisms 123 Int. J. Environ. Sci. Technol. (2013) 10:199–208 201 at the same time. Using multiple separate control programs Experimental objectives for each organism rather than a single control program for all organisms is likely to be significantly more expensive. Experiments were carried out, in 2009 and 2010 in Cam- Additionally, it has long been known that some aquatic bridge, UK, to assess the impact of chlorine on the survival sessile organism populations can be regulated by compe- and growth of bryozoan colonies using an experimental tition for space (Connell 1961). Hence, if multiple organ- set-up that simulated the situation that would result if isms are biofouling a single structure and one organism is chlorine was dosed in the RGF backwash water.