REMOVING SILT FROM CONSTRUCTION WATER RUN-OFF

High levels of silts, fines, clays and colloids can enter natural watercourses as a result of natural events such as heavy rainfall and run-off water, man-made actions such as land drainage and, more challengingly, during major earthworks construction phases such as road building, excavation and remediation of new and old buildings.

Once they have entered into the watercourse, they precipitate and cover the river bed with fine layers of silts and fines, gradually suffocating the life of the river bed. Colloids, on the other hand, can, under some circumstances, agglomerate on the gills of fish and result in fish mortality. In nature, these silts are gradually washed away by successive rainfall events. The challenges facing engineering construction works are made harder by the potential of localised, heavy loads of spill entering water courses, potentially at several locations, with high frequency.

• Silts are generally fine-grained soils that do not include clay minerals and tend to have larger particle sizes than clay (2-4 μm).
• Clays are fine-grained rock or soil material combined with one or more clay mineral (silicates) with traces of metal oxides and organic matter containing variable amounts of trapped water with particle size less than 2μm.
• Fines can be considered to be an overlap between clays and silts exhibiting similarities in particle size and physical properties.
• Colloids (colloidal suspensions) may be regarded as the overall mixture of smaller, non-soluble particle clays and fines (1-0.001μm) dispersed in water. The resultant hydrocolloid is a mix of hydrophilic (negative electrically charged) polymers dispersed in water that do not filter readily or settle and would take a very long time to settle, (days, weeks, months) unlike the larger silt and fine type particles which settle rapidly (seconds, minutes etc.). Therefore, removal of larger particlesized fines, clays and silts can be achieved by appropriate management and use of ‘passive’ systems such as Hy-Tex Silt Fence, sedimentation lagoons, silt bags and Sedimat. To mitigate colloidal suspensions from watercourses an ‘active’ water treatment process is required whereby negative electrically charged clays and fines are ‘charged neutralised’ to allow them to ‘fall-out’ of solution. Although this can be achieved by addition of liquid chemical coagulants, polymers and pH neutralising chemicals, their environmental impact along with health and safety requirements for handling of these chemicals, can be significant.

“With the construction industry starting to pick up again, there are also new concerns. We are under constant pressure from the EU in relation to our water quality and there is no doubt that the construction sector has its part to play. In the past, sediment control or silt retention were not exactly given a high priority in relation to levels of importance on any site. There are varying levels of attention given by the authorities to this matter and it this tends to relate to the proximity of the site to water. The highest attention would be given to a SAC (Special Area of Conversation), and the local fisheries office would closely monitor the sediment run-offs in these areas. Some locations are not as sensitive and there may be more flexibility given to contractors in these areas, however, this is coming under more scrutiny as time passes. On any construction site, water is never too far away and the obligation of clean water running off the site lies squarely with the contractor. Before the downturn, there were no companies providing silt control products, leading to situations where people ‘did their own thing’ with bales of hay or indeed whatever was to hand, but these methods were crude and ineffective. Thankfully things have moved on and there are companies providing solutions to the whole area of sediment control, which will indeed be a big relief for companies who are trying to conform to water quality regulations.”

January/February 2015 CONSTRUCTION 55