## 3 Decentralized Treatment Systems
### Rain and Stormwater Harvesting and Treatment  
The ancient practice of rainwater harvesting is now sporadically employed worldwide to mitigate more or less serious problems connected with water supply, and different levels of advanced technology are associated with it, as illustrated in the recent review paper by [PERSON] et al. This paper examines the existing state of the art in rainwater treatment, the characteristics of the different physicochemical treatment options with emphasis on membrane technologies and the recent developments in biological treatment options, providing an overview of recent developments and trends in the field of rainwater harvesting [119]. In spite of the acknowledged advantages of rainwater and stormwater harvesting, the implementation of harvesting systems is significantly influenced by economic and local law restrictions. Stormwater harvesting and reuse in urban environments is emerging as an alternative water source, despite human pathogens in stormwater that may represent a hazard to public health. The literature data about the chemical and microbial characteristics of stormwater, the possible treatments trains according to the end-use and the pertinent costs have been recently reviewed by [PERSON] et al., providing useful information to local water authorities for increasingly necessary stormwater management in urban areas [122]. Among the possible low-cost strategies, polyester blankets, also known as polyester batting, are easy to obtain and can be used as filter beds. An acrylic blanket (Acrylon) was used as a filtering medium in compact equipment (0.45 m\({}^{2}\)) for rainwater collection and treatment by [PERSON] et al. The equipment, which can be adapted to residential and industrial construction, was tested at a company located in the industrial city of Istaborai (Rio de Janeiro). Six layers of 2.5 cm acrylic were placed, for a total of 15.00 cm of thickness, and the equipment used for 10 days, 8 h/day. In this experiment, the first flush was included, and water quality monitoring was conducted by analyzing raw and treated water. The color removal efficiency of the equipment showed an average value of around 58.84%, considering the entire period in which the first blanket was used. The treatment met the requirements of the Brazilian standard for the use of rainwater concerning turbidity (less restrictive standard), residual chlorine and absence of coliforms but did not meet the requirements for hydrogen potential and color removal. Analysis on the treated water in a storage tank was also carried out to measure how long after chlorination, conducted in the equipment by calcium hypochlorite, the chlorine concentration would be within the limits for reuse. The results show that even chlorine at low concentrations was enough to inhibit the presence of microorganisms. Improvements like the increase in the filter bed, isolation of treated water from the filter and discarding the first flush (as usually conducted) should help in producing clean water in developing or underdeveloped countries [150]. Multilayer blue-green roofs (MBGRs) have been widely recognized for their high capacity for reducing runoff generation from rooftops, and their additional storage layer enables for collecting water, which could be reused for different purposes. The potential benefits of a multilayer blue-green roof in terms of water collection and reuse, according to national and international regulations, was investigated by [PERSON] et al., by using irrigation and rainfall inputs and performing water quality analyses on the outflow, which presented lower suspended solid and heavy metal concentrations than from a traditional roof. On the other hand, COD in the outflow exceeded the limits defined by Italian regulations (125 mg/L) for water discharge or reuse. This high COD value was partially due to the high residence time in the storage layer. The outflow can be used mostly for irrigation of the MBGR itself or for small urban gardens.