Water resulting from construction sites, civil engineering or special works, have to be considered by French standards, which require treatment and drainage of work site water. Judgment of 2nd February 1998 about collections and water consumption as well as emissions of any nature of installations, refers that constraints of Residual Maining dry Solids (for a rejection > 15 kg/d) are as follows:
The judgment also establishes constraints about pH level (5.5 to 8.5), DC0 and DB05, phosphorus, nitrogen, hydrocarbons, heavy metals, which may be supplemented by specific prefectural judgments.
These waters resulting from construction sites are usually altered by drillings, excavations phases charging water with suspended solids (mix between extra-fines, colloids, clays, silts) and contaminating them with hydrocarbons (fuels, oils, solvents). In some cases, and especially in contact with concrete, pH of these work site waters increases because of concrete laitances and we also could find dissolved polluting substances, which are incompatible for a rejection in a natural environment.
Rejection of these waters requires therefore upstream treatment to clarify them and to minimize the impact on the environment. This is achieved through a system of water treatment.
PRESENTATION OF THE PROJECT
MS was asked by Vinci Construction in 2012 to meet this need of work site waters treatment for the project of the Odéon Tower construction in Monaco. With its 170 m high and its 49 floors, the Odéon Tower will be the highest building of the Principality, marking the return of high-rise construction in this cramped territory of Monaco.
Designed by architect Alexandre Giraldi, this prestigious tower saw its foundations beginning in September 2009 and structural work in February 2012, which is now completed. It’s time to convert the interior and exterior of the tower, with a final delivery scheduled for the second half of 2014.
This atypical project, which has to be completed within a very short period, has highlighted several technical constraints including the smallness and the location of the site. These different constraints led to the establishment of a rigorous environmental policy. The Principality has imposed the implementation of a work site water treatment plant to clean up sewage waters from the excavation of materials.
Vinci Construction asked for MS company, which thanks to its know-how in solutions of liquid/solid separation, has guided the argumentation with the team of Vinci and suggested this innovative concept: design, manufacture and install a mobile work site water treatment plant, which will be optimally set up on the construction site.
TECHNICAL CHALLENGES AND PROVIDED SOLUTIONS
The study in our laboratory of sewage waters from the excavation of materials, underlined a maximum concentration of 25 g/l with 10 gr/l after desanding. The objective of this equipment was to isolate the suspended solids and to recycle water with a maximum post-treatment rate of 30 mgr/l, for a compatible rejection in environment.
As part of the Odéon Tower work site, a treatment of pH by acid adding was also necessary due to the presence of concrete laitances in the waters.
This modular work site water treatment plant that can accept a flow of sewage water up to 60 m3/h containing up to 600 kg/h of Residual Dry Solids after desanding, include the following process:
- Buffer tank to absorb and smooth the flows and the concentration of the water feeding the settling unit.
- Settling unit, using flocculation & coagulation, with a regulation of the incoming effluent (re-concentration or dilution).
- Dehydration of the concentrated sludge
- Correction of the pH (by adding acid)
- Additional filtrations, with a target of 30 mgr/l of residual dry solids in the clarified water.
The plant is made of 4 pre-assembled and pre-wired blocks (desanding, buffer tank, thickener and technical room). The 2 double - blocks "desanding + buffer tank" and "thickener + technical room" can be installed as per several layouts depending on the surface available on the jobsite.
As the incoming water of the Odéon Tower work site contained sand particles and gravels +4mm, a scalping and desanding unit was required to optimize the flocculation and avoid an overconsumption of flocculant, the early wearing of the sludge pumps and the blockage of the thickener.
Water was first scalped on the 1st compartment of a vibrating screen (fitted with grids # 5 mm) to eliminate the gravels and the waste components. The passings were transferred into a sump feeding a cyclone to cut at 60 µm.
The cyclone underflow fed the 2nd compartment of the vibrating screen (fitted with grids # 600 µm) for dewatering of the recovered sand. Then, the desanded water (cyclone overflow) was transferred to the buffer tank. The scalping/desanding unit was designed to reach a peak flow of 100-120 m3/h.
Buffer tank and thickener feed
As the plant may be fed from several pumping wells, flow and consistency of the incoming water may vary a lot. In order to permit and optimize the flocculation, it was important to smooth the flow and the consistency before feeding the thickener.
The buffer tank (capacity of 30 m3, i.e. 1/2 of the hourly flow) is fitted with:
- A first pump to recirculate the effluent and therefore avoid any natural settlement of the dry solids in suspension
- A second pump (with frequency inverter) to feed the thickener at a nominal flow (controlled by level sensor and flow-meter on the feed piping)
The smoothed effluents coming from the buffer tank fed a box fitted with baffle plates in which they were mixed with a flocculant solution (prepared by the "DOSAFLOC") and coagulant, and were then transferred to the central feed shaft of the thickener. Flocculant dosing is monitored thanks to the automatic control of the flocculation speed made by the "CONTROLFLOC".
The sludge settled quickly at the bottom of the thickener where it was concentrated thanks to the slow movement of the scraper, before being pumped to the filter press and/or for recirculation. The clarified water overflowed into a storage tank. The peripheral overflow channel was fitted with a siphoïd wall in order to trap the floating materials and oils.
The sludge was concentrated at the bottom of the settling unit up to a consistency of 500 - 600 gr/l. In order to change this semi-liquid waste into a solid waste (therefore easily disposable without any specific means), a filter press (capacity of 200 to 600 kg/h of dry solids) was built-in to the plant. Located inside the technical room, this filter press was fed from the sludge pump closed to the settling tank. Pressing cycles were fully automated.
Treatment of pH
Because of concrete laitances, the clarified water required a pH treatment. Usually, water is rather basic. In order to decrease the pH, 2 technologies could be proposed: acid or CO2 dosing. On the work site in Monaco, technology with acid was used, including acid dosing directly into the clarified water storage tank and mixing thanks to an agitator.
The average residual dry solids content of the overflowing recycled water was of 50 mgr/l with a guarantee at 80 – 100 mgr/l. The target being to reach a maximum of 30 mgr/l, an additional filtration was necessary. Clarified water was pumped from the storage tank and sent through a sand filter filled with a grading sand dedicated to trap the possible micro-flocs, and then sent back to the storage tank (closed loop system).
Supervision and measurement sensors
The plant was equipped with a kit of sensors and a supervision panel PC enabling to issue statistics of the incoming and discharged water. The main measured parameters were the incoming water flow, the volume of treated water, the temperature, the turbidity and the pH of the discharged water.
DELIVERY AND ASSEMBLY ON SITE
For this project, the whole plant was pre-assembled and pre-wired in our workshop, thus ensuring a fast and easy erection, as well as a more efficient commissioning on site in Monaco.
Starting from our workshop in Veyre-Monton (63)
Handling on the work site
Implantation on the work site
In the form of rental offer, including supply of the station and related services, this plant allowed to value and treat during 15 months effluents from work site of the Odéon Tower. Today disassembled and reconditioned in our workshops for a new lease or acquisition, this solution has highlighted this desire to respond to environmental issues and water treatment. The extent of this project, its technical constraints and innovative appearance made it a flagship project in France and in our company.