Bloom prevention and control.

Abstract

Harmful algal blooms (HABs) are a serious and growing problem to many sectors of society, including the desalination industry. The many problems that HABs present for seawater reverse osmosis (SWRO) desalination plants include: 1) the production of dangerous toxins that have the potential to contaminate treated water; 2) high algal biomass that clogs intake filters; and 3) contributing to biofouling of equipment and SWRO membranes. It is important to limit the impact from HABs by preventing blooms from reaching SWRO plants in the first place, while also reducing their effects in the event that ingress to the plant has occurred. Many of the management actions taken to respond to HABs can be termed mitigation – i.e., dealing with an existing or ongoing bloom, and taking whatever steps are necessary or possible to reduce negative impacts. Mitigation strategies can be classified into two categories, precautionary impact preventions and bloom controls (Kim 2006; Anderson 2004). Precautionary impact preventions refer to monitoring, predictive, and emergent actions - essentially actions taken to keep HABs from happening or from directly impacting a particular resource. Several problems are immediately apparent in this regard. For one, we do not have all of the knowledge we need about why HABs form in many areas, so it is obviously difficult to regulate or control those factors. This argues for substantial and sustained research on all aspects of HABs, including their ecology, physiology, and oceanography. All too often managers and agency officials view these topics as fundamental or basic science issues that have little direct practical utility, but in reality, such knowledge is essential for the design and implementation of effective prevention strategies. Another problem that arises with the concept of HAB prevention is that even if certain environmental factors are known to influence the population dynamics of a specific HAB organism, there are limitations on what can feasibly be done to modify or control those factors. It might be known that a particular HAB is strongly influenced by the outflow of a river system – that it is associated with a buoyant coastal current, for example - but are unlikely to be able to justify the alteration of that river flow solely on the basis of HAB prevention. As discussed below, it is nevertheless important to factor the possible impacts on HABs into large-scale policy decisions on such topics as pollution reductions or alterations in freshwater flows in response to agricultural and drinking water demands. Obvious examples of impact prevention in the context of desalination are pretreatment strategies that remove cells and the organic compounds they produce. These are described in Chapter 9. In effect, these strategies are used to cope with HABs and to manage around them. The question often arises, however, as to whether it is possible to be more pro-active. Can something be done about blooms before they happen, or can something be done to destroy or suppress them while they are occurring? These questions highlight the “control” aspects of HAB management. Bloom control is both challenging and controversial. The concept refers to actions taken to suppress or destroy HABs, intervening directly in the bloom process. Curtailing or suppressing the duration and magnitude of a HAB through physical, chemical, or biological intervention are potential approaches, but this is one area where HAB science is rudimentary and slow moving. Anderson (1997) highlighted the slow research progress on bloom control, in contrast to aggressive policies to control pests and nuisance species in terrestrial agriculture. A number of reasons were listed for the reticence or reluctance of scientists and managers to explore and implement control strategies. These include: • HABs are complex phenomena in highly dynamic environments. Many are large, covering thousands of km2. Control strategies would be massively expensive and logistically challenging. HABs are caused by algae from many phylogenetic clades (see Chapter 1), including eukaryotes (armored and unarmored dinoflagellates, raphidophytes and diatoms, euglenophytes, cryptophytes, haptophytes, pelagophytes, and chlorophytes) and microbial prokaryotes (cyanobacteria that occur in both marine and freshwater systems). Given this biodiversity, no single strategy or approach to bloom control or suppression will apply to all harmful algae. • HAB phenomena remain poorly understood, i.e.,“we can’t control what we don’t understand”. • Few, if any, countries have government agencies with the mandate to conduct research or to implement strategies to control marine “pests”. • The solutions may cause more damages than do the HAB problem being treated. Each of these arguments has a counter argument, as discussed in Anderson (2004), but the bottom line is that progress on bloom control has been slow, with advances being made by only a few countries. The challenge is even more significant when viewed in the context of a desalination plant. In the discussion that follows, traditional and emerging technologies in the field of HAB mitigation and control are summarized in the context of their applicability to HAB risk management at SWRO desalination plants. In doing this, it is recognized that desalination plants are unlikely to undertake any large-scale bloom control or suppression strategies outside their plants, given the cost, logistics, and uncertainty of such efforts. It may be that bloom control would be considered at a small scale within an embayment or intake lagoon, and thus it is important to know the various approaches that have been attempted in different systems. This will also help operators address a very common question from the public, or from plant management – “Is there anything we can do to control or stop this bloom before it enters the plant?”