Advances in Water Treatment and Enviromental Management

228 WATER TREATMENTstreams of low water content, with minimal rates in the warm seasons up to 30m3/s, low rates of flow, below 0.3 m/s, and specific river discharge per one urbaninhabitant below 0.03 l/s. Urban reservoirs with low fiowage, with specific watervolume less than 1000m 3 per 1 ha of built-up adjoined territories, can also beincluded in this category.Traditionally SS loads can be reduced by construction of a partially separate systemwith partial bypass to the municipal sewage treatment works, or sedimentationlagoons at swere outfalls, and in exceptional cases filters can be provided for tertiarytreatment.Treatment of NP discharge gives relatively small BOD reduction, only about 3–6 %of the raw municipal sewage BOD, but capital investments for this purpose arecomparable with that for municipal sewage treatment works. Capital investmentseven for simplest NP discharge treatment, such as lagoons, when calculated perone tone of captured organics, are 2–3 times higher than for tertiary treatment ofmunicipal sewage.Such high specific costs, especially capital ones, for NP discharge treatment unitsare primarily due to the NP irregular periodicity.At present the main sources of urban NP discharge pollution are sedimentingaerosols, destruction of road coverings, tyres attrition, and soil erosion. If erosionproducts were reduced to average values, then polluting loads of the aerosolcomponent would amount to 58% for cities and to 78% for towns. Products of roadcovering destruction amount to 10–25 %.Investigation of dispersive and dynamic characteristics of urban aerosols obtaineddata on SS concentration in NP discharge produced by the design rain under variousdegrees of dust content in the air (see Table).The table data demonstrate that reduction of dust content in the lower atmosphere,primarily in areas with heavy polluting industries, can alone cut SS wash-off byseveral times. Such an approach is economically viable.According to our calculations capital costs of 1kg of dust removal from the air isabout 85-fold cheaper than 1 kg of SS removal at a partially separate system unitfor NP discharge treatment and about 16-fold cheaper than removal in lagoons.Prognostication demonstrated that at the normative air dust content 0.15 mg/m,prevention by simple measures of soil wash-off from lawns and open areas, aridhigher durability of road covers can reduce by 30% the total wash-off of pollutantscaused by the design rain.Application of environment protecting and technical measures reducing technogenicpolluting loads in urban NP discharge will allow cuts in treatment costs of 33–70 %,and in some cases, with a large discharge of the stream, will completely obviatethem.ConclusionsIntroduction of limits on build up of fine-grained particles on urban territories is apromising way for environmental improvement as a whole in all media, includingwater bodies. If existing environmental protection measures do not reduce total