EAs del Gobierno Central:

Combined sewers that carry both sanitary sewage and storm water runoff during rainfall events service most of our cities. Most sewage treatment plants have been sized to treat only dry weather flows of sewage. During storm events the combined sewer overflows (CSOs) lighten the load on treatment plants by discharging untreated sewage into local streams. Most CSOs are located in heavily populated urban centers and are universally considered to be an urban problem. The concept of CSOs is that storm water was expected to dilute the sanitary flow, but it was then found to have a considerable pollution load of its own. Effective measures described here are source control and off- line storage.

Combined sewer overflow source control

Source control reduces the quantity of pollutants entering the system. This includes control of illicit connections, street sweeping, catch basin cleaning, and stormwater management measures that reduce or delay the volume of runoff entering the system. Measures to conserve water used in households will also reduce loads on treatment plants.

Off-line storage of combined sewage

This type of storage involves the containment of combined sewage that normally would overflow and discharge to receiving rivers. Storage facilities are usually large underground tanks or tunnels. When flow capacity is once again available within the system then the stored combined

sewage is conveyed to the treatment facility. The controlled activation of large-dimension pipes for combined detention and transport purposes is a promising storm water management approach. With this strategy, the total load of drainage overspills and treatment plant outflow can be minimized. It requires the real-time control of weirs, pumps, in- and off-line detention facilities based on a computer-model which is fed with rainfall and pipe-flow measurements. However, the main purpose is rather pollution reduction than the re-naturalisation of a river flow regime.

5.3.8 Emission controls

The existing channel network is frequently damaged because of consolidation and overburdening of soil, aggressive materials in sewage and poor quality materials, from which pipes and seals are made. Regular maintenance and reconstruction of the network is necessary for reducing the level of emission that through subsoil drains into streams. The measures are necessary to prevent pollution of the environment and groundwater. Most advantageous are areas near the streams and areas rich in groundwater. The measures enhance the physico- chemical state of waters.

However, the sewage system is not the only source of emissions from the urban areas. Emissions result from flushing the pollution from the air due to traffic, spillage of pollutants in discharge sites, leakage from abandoned disposal facilities, non-restored industrial plants, polluted land in the courtyards of industrial plants etc.

Emissions from the air are mitigated with measures for air protection and emission reduction

The problem is especially pronounced in areas with siliceous geological formations that are unable to neutralize the acid rain phenomenon. In such areas reduction of emissions from the air is a prerequisite for revitalisation.

Emissions in the air due to congestion depend on the technological development of vehicles and traffic organization. With an extended safety green belt among the roads, railways and the stream, the adverse effects can be reduced. Additionally, a better traffic organization and reduced traffic jams along the streams (traffic control) lead to the reduction of impacts. A direct pollution inflow at the outflow from roads is reduced by reservoirs and wetlands.

Spillage of pollutants in discharge sites

Discharge sites and manipulative areas can be covered, so that the contact between the precipitation water and pollutant areas is disabled. The area of the discharge site is designed as an interceptor bowl, so that in case of accident the pollutants cannot spill outside the covered area. Water from the roofs is directed into groundwater and surface reservoirs, and in that way a favourable influence upon the hydrological regime is achieved.

Leakage from abandoned disposal facilities

Disposal facilities must be recorded and remedied, especially the facilities in direct contact with a water body.

Non-restored industrial plants, polluted land in the courtyards of industrial plants

The procedures taken are the same as applied in abandoned disposal facilities. A special problem presents the polluted upper layers of soil.

5.3.9 Increasing low flow

The basic measure is to increase the groundwater storage, which recharges the watercourse; direction of surface waters from the watercourses rich in water, designing water bodies of standing waters (reservoirs and wetlands), building small local treatment plants in headwaters and separation of used waters and their treatment in wetlands. The measures have a favourable effect on the hydrological regime, indirectly also on the entire ecological conditions in the watercourse. Low flow is the basis for survival of habitats in dry conditions, it enables a bigger dilution of pollution, smaller temperature changes and thereby an improvement of the entire ecological state.

Increase of groundwater storage

Besides other measures recharging the groundwater, the non-polluted water is diverted from the roofs through treatment plants into groundwater.

Diversion of surface water from watercourses rich with wate

Bifurcations with a gravitational water drainage are built. Special emphasis should be given to mill water constructions as artifical water bodies.

Building bodies of standing waters (reservoirs and wetlands)

Reservoirs and wetlands are arranged in the green urban areas, as well as ponds.

Building small local treatment plants in headwaters and separation of used water and its treatment in wetlands

Waste water is separated onto black and grey waste water. Grey water can be sufficiently treated with simple treatment plants in order to recharge the watercourse. The disadvantages are mainly large daily fluctuations that are diminished by wetlands and reservoirs.