First rain water treatment plants

First rain water treatment management is one of the primary objectives aimed at the protection of the receiving water bodies. Such water, in fact, constitutes the vehicle through which polluting loads are discharged into water bodies with consequent environmental impact. These pollutants consist of heterogeneous mixtures, un-dissolved substances, and suspended colloids, including metals, organic and inorganic compounds.

First rain waters need appropriate treatment in order to guarantee the protection of aquatic ecosystems in correspondence with the quality objectives set by European directives 2000/60/CEE (Water Framework Directive) and 91/271/CEE (concerning urban wastewater treatment).

In urban areas, sources that cause the change of meteoric storm water quality can be divided into two categories; diffuse sources on the territory (roadways, parking lots, etc.) and point sources, like infrastructure junctions and production sites, in which the typology of polluting cargo is closely related to specific activities performed. Regarding the diffuse sources, as documented in literature, numerous monitoring campaigns were carried out for the characterization of the first rain water, in order to determine both the process of formation and the accumulation of polluting substances, as well as the subsequent phase of transportation inside of the unitary and separated types of drainage systems.

Regulations

The art. 113 of Legislative Decree 03 April 2006 no. 152 part III (Dispositions on the protection of waters from pollution) asserts that the waters are legislated. The communitarian directives no. 91/271/CEE (Treatment of urban waste water), and no. 91/676/CEE (meteoric storm waters and first rain waters), both received from the Italian state, assert:

 “....... in order to prevent hydraulic and environmental risks, the regions, by authority received by the Ministry of the atmosphere and of the protection of the territory, legislate and put into effect:

a) the form of control of the meteoric storm water drainage coming from separate sewage systems; 

b) ……, etc.”.

The law of the Lombardy region of 24 March 2006 no. 4, relative to the “Legislation of the drainage of first rain waters and storm waters of external areas” in performance of article 52, comma 1, letter a) of the regional law 12 December 2003, no. 26 (Official Bulletin of the Lombardy Region, 28 March 2006 no. 13, 1st regular supplement) sanctions (referring back to some articles):

Art. 3 (first rain waters and storm waters subject to regulation) 

The formation, the conveyance, the separation, the collection, the treatment and the drainage of first rain waters are subject to the dispositions of the present regulations in case such waters come:

a) from the draining of surfaces that extend up to 2000 m2, calculated to exclude coverings and green areas, pertinent constituents of buildings and installations in which the following activities are carried out: oil industry, chemistry industries, metal treatment and finishing, leather tanning and dying, production of paste paper (of paper and cardboard), production of pneumatic devices, textile companies that execute press tinctures and finishing of textile fibers, production of concrete, multi-purpose areas, machine-shop trucks, auto bodies;

b) from the draining of the pertinent surface constituents of buildings and installations in which refuse deposit activities are carried out, collection centers and/or transformation centers, junk warehouses and vehicle warehouses destined for demolition;

 

c) from the draining of surfaces used for the storage and distribution of fuel and related operations, at the points of sale of service stations for motor vehicles;

 

d) dfrom the draining of surfaces that are specifically or occasionally used for the storage, transport, drainage, the effusion and the general distribution of the substances on tables 3/A and 5 of attachment 5 of the Legislative Decree no. 152 part III dated 3 April 2006.

Art. 5 (systems of collection and conveyance of first rain waters and storm waters)

1) All the surfaces mentioned in article 3 must be impermeable.

 

2) The first rain waters and storm waters, that are to be returned to the body of surface water that is on the ground, or in superficial underground layers, must be routed to appropriate water tight collection tanks, dimensioned so as to withhold altogether at least 50 cubic meters per metric acre of drainage surfaces (first rain tanks as follows).

 

3) The meteoric storm waters must be routed to an appropriate network of collection and conveyance, fortified, in the cases mentioned at codicil 2, with a feeding system into the first rain tanks that automatically excludes them when full; the network must be proportioned based on the meteoric events of short duration and of elevated intensities characteristic of every zone, and at least assuming, however, that the event takes place in fifteen minutes, and that the coefficient of influx to the network is equal to 1 for the draining surface, and 0.3 for those permeables of any type adjoining them, excluding from the computation uncultivated surfaces and the those of agricultural use.

The regulations of the Lombardy region by now are adopted by nearly all the Italian regions.

The prescribed sanctions for whoever does not observe such legislative dispositions are regulated by the Legislative Decree 152/2006 part III art. 133 codicil 9 with a fine from 1500 to 15000 euro, and for whoever does not respect art. 113 of the same decree codicil 3 the enunciated penal sanction in Legislative Decree 152/2006 part III art. 137 codicil 9 that prescribes the arrest from 2 months to 2 years.

Description and Operation

The treatment of first rain waters is performed by a system of grillwork, sediment filtration, and oil separation. The first rain waters are conveyed through a water trap by-pass (to separate first rain from second rain waters) into the appropriate said tanks “First Rain Tanks”. The treatment system includes 3 distinct phases:

1. Separate through a spillway trap the first meteoric waters that become polluted from the second ones.
    

2. Temporarily accumulate the first very polluted meteoric waters because they wash roads and large areas, in order to allow, during their temporary storage, the sedimentation of the solid substances.
    

3. Convey the waters that are temporarily stored at one treatment unit for the separation of hydrocarbons.

In the current practice, the first rain waters are separated from the subsequent (second rain) and dumped back into the treatment unit (NS Oil Separators) through a basin of buried accumulation, that has the ability to contain all of the meteoric storm water resulting from the first 5mm of rain fallen on the pertinent draining surface of the system.

The basin is preceded by a separator trap that contains a weir on the inside. The waters of the second rain skim off the weir as soon as the water surface in the basin catches up to the level of the threshold of the weir.

An emptying pump is installed in the basin, and is automatically activated from the switchboard through a microprocessor that amplifies the signal of a rain detector probe installed on the intake pipe of the trap.

At the end of the precipitation, the probe sends signals to the switchboard which starts the pump running again after an interval of time equal to 96 hours less than the previous emptying time.

If during such interval a new precipitation begins, the probe resets the waiting time to zero.

Once the basin is emptied, the level switch deactivates the pump and the system returns to wait mode.

Terms of the contract

The Treatment System of First Rain Waters of the MUSILLI W400 series, is manufactured with monolithic accumulation reservoirs prefabricated in reinforced vibrated concrete to high resistance verified for street cargos of the antiseismic category 1, complete with sections for the sand separation, a bypass trap prefabricated in reinforced vibrated concrete, connection grafts in pvc, a cover slab prefabricated in carriageable reinforced vibrated concrete verified for cargos of the 1st antiseismic category with inspections bumper to bumper and D400 class valves. The reservoirs are equipped inside with a rain sensor, anti-reflux valve, submersible electric pump for raising storage waters, complete with automatic connection feed to the delivery pipe, electric control panel, and protection, integrated to programmable electronic logic (PLC). The system is dimensioned according to the law of the Lombardy region no. 26 of 12/12/2003 art. 52 codicil 1 (Official Bulletin of the Lombardy Region, 28 March 2006, no. 13, 1st regular supplement), and according to the Legislative Decree n. 152 of 4/3/2006.

The system is constructed by the company in possession of Business Quality System certification UNI EN ISO 9001:2008 certified ICMQ.

In supplement it is possible to provide the oil separation plant MUSILLI W200 series system, made with reservoirs prefabricated in reinforced vibrated concrete to high resistance in reinforced vibrated concrete verified for street cargos of the 1st antiseismic category, and equipped with cover slab prefabricated in reinforced vibrated concrete verified for street cargos of the 1st antiseismic category with inspections bumper to bumper and valves in D400 class cast iron, coalescence filter and floater stopper. The oil separation system is dimensioned according to regulation UNI EN 858 part 1 and 2.

Calculation of the pluvial capacities

The quantification of the rain capacities is one determining factor for the dimensioning of the collectors and the relative treatment work. Such quantification can be calculated and verified through various theories and models, described as follows, which refer to our technical study.

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• Theoretical Treatment for the determination of the meteoric influx with consequent hydrological study of intense rains.

• Calculation of the meteoric capacities produced from the city surfaces to the sewage collectors.

• Calculation of the volume of first rain waters:
   

  a) Method using run-off time;

  b) Method using first rain time;

  c) Method using first rain height.

As an example we present here as follows, a useful table for the fast determination of the volume of first rain waters to be drained in connection with the surface area waters, obtained using the method of the first rain height.