One of the pools built for sedimentation of the solids in suspension in the waste water from the crushing process was not meeting its objective efficiently. Consequently, the concentrations of the solids suspended in the final effluent were above the maximum permitted threshold according to the relevant regulations.

The problem was caused by a flaw in the pool design, as this was not appropriate to ensure the retention time needed to allow the deposits to settle.

Specifically, the concentration of the suspended solids in the effluent was not considered as an important factor when the pool was designed. What is more, no protocol was established to regulate the activation of the pumps that discharge the water into the pool from the Northbound channel, nor was the use of any chemical agent considered (for example, flocculants) to facilitate the precipitation of the solids in suspension.

To optimise the pool system, reduce the concentration of solids in suspension and comply with the maximum permitted limit, the following measures were implemented:

• The pool was divided into two compartments by placing stones which simultaneously functioned as a filter.
  • In the first compartment, which receives the water from the sedimentation bath at the crushing plants, the flocculant is added, which enables the greater part of the sediments to be decanted. This compartment is cleaned regularly, as the sediment accumulates.
  • After the sediment flocculation, the water exits through a small sluice to one side and flows through the second compartment until it reaches the pumping point. As this is a longer route, the sediments are captured. Cleaning of this zone is conducted according to the extent of sediment saturation. In this second compartment, a dipstick was placed to determine the water level and monitor the pumping of the water to the diversion channel.
• A dosing dispenser was fitted to dispense liquid flocculant into the channel outlet for the water from the crushing and rinsing processes at the crushing plants.

As a result of this process, the silt is deposited in the Disposal Site allocated and the water is pumped to the North Diversion Channel, where sampling is conducted twice a month to check that regulations are being complied with, as well as to monitor the effectiveness of the process.

The results of the samples taken since the pool sedimentation system was overhauled by dividing the pool into two compartments, adding flocculant and placing a dipstick to determine the water level, confirm that that the pool is now more efficient. With the solutions adopted, the retention time and effluent quality have been improved, and the concentrations of suspended solids are well below the maximum limits according to regulations.