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Ian Heffernan - Civil Engineers in Arklow, County Wicklow

Nutrient Removal

Overview

Ian Heffernan and AssociatesĀ can design the wastewater treatment plant to incorporate nutrient removal on request. The nutrients in question are Total Nitrogen, Ammonia and Total Phosphorus and the required levels of treatment must be specified.

Why nutrient removal?

Nitrates

Nitrate in excess concentrations in water may constitute a risk to human health and the environment. Nitrogen enters on-site wastewater treatment systems mainly as organic nitrogen, which means the nitrogen is part of a large biological molecule such as protein.

High-performing wastewater treatment plants incorporating primary and secondary treatment should convert a proportion of the ammonia present in the wastewater to nitrate. Bacteria and other microbes oxidise or mineralise the organic nitrogen to ammonia, which is further oxidised to nitrites and nitrates.

The discharge of wastewater containing nitrate to percolation areas has generally been considered acceptable as de-nitrification (the conversion of nitrate to nitrogen gas) will occur in the soil along with nitrate removal by uptake in plants.

However, new EU legislation concerning nitrate-sensitive zones means thatĀ  the discharge of nitrate-containing effluents to soil or water is becoming increasingly restricted. Some form and degree of nitrogen removal is becoming an increasingly common requirement for wastewater treatment systems.

A Groundwater Protection Response compiled by Ireland’s Environmental Protection Agency (EPA) and te Geological Survey of Ireland in 1999 states that:

“Where nitrate levels are known to be high or nitrate loading analysis indicates a potential problem, then consideration should be given to the use of treatment systems that include a de-nitrification unit.”

Phosphorus

Phosphorus is an essential element of life, but too much of it in the wrong place and it becomes a pollutant. In natural waters the low solubility of natural phosphates means that phosphate levels are low and phosphorus is usually the limiting nutrient. Excess phosphorus from any source thus leads to abundant growth of plant material in water, as oxygen is used up by decaying matter, and waterways choke on their own production.

This process is known as eutrophication and is a natural process by which stagnant water bodies slowly turn into bogs. Enrichment speeds up this process and the sight of open lakes turning into bogs, as plants encroach on the open water, is not uncommon in Ireland.

This side-effect of phosphorus is one of the major reasons why phosphorus in detergents is controversial, although fertilisers are another source of phosphorus in surface waters. Human and animal wastes together are the biggest source of phophate in the water. This is why proper treatment of sewage is crucial to solving the excess-phosphorus problem.

Biological treatment

Biological treatment of wastewater involves either aerobic or anaerobic processes to remove organic matter. For phosphate removal in biomass, an anaerobic zone must be incorporated into the activated sludge basin.

Although most of the phosphate can be removed by suitable biological treatment, the remaining phosphate must be removed by chemical processes.

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