What is pH and Why does it matter ?

The Short Answer

pH is a measure of how acidic or alkaline a substance is. It is particularly important in environmental protection because changes in pH can affect water quality, aquatic life, soils, plants and the behaviour of pollutants.

What does pH mean?

The pH scale normally runs from 0 to 14:

The scale is logarithmic. This is important: a solution at pH 5 is 10 times more acidic than pH 6 and 100 times more acidic than pH 7.

Why is pH important environmentally?

Natural ecosystems generally operate within relatively narrow pH ranges. A significant change can cause several problems:

Aquatic life
Fish, aquatic insects, amphibians and microorganisms can be sensitive to pH. Very acidic or alkaline water can cause direct physiological stress and, at extreme levels, mortality.

Toxicity of pollutants
pH can change the chemical form and mobility of contaminants. For example, the toxicity and availability of metals such as aluminium, copper and zinc can be strongly influenced by pH.

Nutrient availability
pH affects how nutrients such as phosphorus and nitrogen behave in soils and water. This can influence plant growth and contribute to problems such as eutrophication.

Soil health
Very acidic soils can increase the mobility of some metals and reduce the availability of important nutrients. Very alkaline conditions can also make certain nutrients less available to plants.

Wastewater and industrial discharges
Discharges with an unusually high or low pH can damage receiving waters. Consequently, pH is one of the basic parameters routinely monitored when assessing wastewater and water discharges.

Typical pH limits for water discharges

Typical Limits for discharge to surface water:

Discharge to Controlled Water (Surface Water/Groundwater)

• Minimum pH 6

• Maximum pH 9

Discharge to Foul Sewer

• Minimum pH 6

• Maximum pH 9

Why pH is particularly relevant to concrete wash water

This is especially important for concrete wash water, because fresh cement and concrete residues are highly alkaline. Concrete wash water can therefore have a very high pH, often around pH 11–13 or higher, depending on its composition and dilution.

If untreated concrete wash water enters a stream, river, drain or surface-water system, it can cause a rapid local increase in pH. This can harm aquatic organisms and alter the chemistry of the receiving water.

This is why pH should be regarded as a key environmental-control parameter when managing concrete wash water. Treatment systems should normally aim to prevent highly alkaline water from being discharged to the environment.

An important point

pH on its own doesn't tell you whether water is environmentally safe. Two samples can have the same pH but very different environmental risks. For example, environmental assessment may also require measurements of:

• TSS (Total Suspended Solids)

• turbidity

• alkalinity

• electrical conductivity

• metals

• hydrocarbons

• nutrients such as phosphorus and nitrogen