Introduction and Agricultural Practice
2.4.1 Introduction and Agricultural Practice
Constructed wetlands have become widespread as an alternative and cost-effective method of treating wastewaters including agriculture-related wastewater streams (Harrington et al. 2005; Kivaisi 2001; Scholz and Lee 2005; Sievers 1997; Stone et al. 2002). As opposed to traditional mechanical and chemical treatment tech- niques such as sludge treatment plants or trickling filters, constructed wetlands implement only processes that occur naturally. The effective treatment of swine wastewaters (also called pig slurry in Europe) has been studied in greater depth over the past two decades, with more and more processes being examined and scrutinized.
The majority of research originated in the USA, where government agency guidelines were published for the construction and operation of constructed wet- lands for various applications, but predominantly for domestic wastewater and storm water treatment (USEPA 1988, 2000). However, with respect to the com- plex nature of diffuse agricultural pollution, these guidelines were found to be inadequate (Sievers 1997; Stone et al. 2002, 2004) and resulted in mediocre over- all treatment results due to the absence of a pretreatment stage promoting sedi- mentation or nitrification. Particularly livestock waste and wastewater are difficult to treat sufficiently by constructed wetlands due to their high concentrations of pollutants including suspended solids, nutrients, and bacteria (He et al. 2006). Moreover, an increasing number of livestock on farms and high production rates result in very high volumes of wastewater that prove to be a challenge to con- structed treatment wetlands.
In the European Union, the Nitrates Directive 91/676/EEC (EEC 1991) has put pressure on piggeries due to the restrictions placed on land spreading of corre- sponding wastewaters at certain times of the year. Considering that suitable farm- land for spreading is limited and other landowners are often unable to spread pig- gery waste as fertilizer, expensive and high-maintenance equipment is often purchased that can put great financial burdens on piggeries and farmers. The holis- tic approach of using constructed wetlands for the treatment of wastewater is therefore rapidly becoming a far more appealing approach to many farmers. How- ever, constructed wetland acceptance will be hindered without greater understand- ing of the processes involved and guidance to their construction, operation, and maintenance.
74 2 Wetland Case Studies
The initially widely used guidelines from the USEPA (1988, 2000) were suffi- cient for certain types of wastewaters. Long and narrow systems often yielded respectable results despite high flow velocity measurements. This basic design has progressed to more sophisticated wetland system designs adopted to cold, temper- ate, and tropical climates (Kantawanichkul 2003; Puustinen 2005). Different de- signs include vertical-flow, horizontal surface-flow, and sub-surface-flow systems (Scholz 2006a).
The basis for the use of constructed wetlands for wastewater treatment is well established and its ‘fine-tuning’ for the use of intense agricultural wastes is ongo- ing. However, some systems such as ICW have produced exceptionally positive treatment results for farmyard runoff and domestic wastewater (Scholz et al. 2007) and are therefore being considered also for swine wastewater treatment.
The aim of this review is to assess the current level of knowledge and under- standing with respect to the treatment of wastewater high in ammonia–nitrogen with constructed wetlands. The review will focus on swine wastewater treatment representing a ‘worst-case scenario’ for wetland designers. The reasoning for further research needs incorporating multidisciplinary and holistic views is also outlined.