How to Measure Sludge Levels in Aerated Lagoons

Why measure sludge levels?

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To ensure optimal performance of municipal wastewater treatment facilities and to comply with the requirements of Quebec’s Ministère des Affaires municipales et de l’Habitation (MAMH), wastewater treatment plant operators must regularly make sure that the sludge level in aerated lagoons does not reach the recommended maximum.

Measuring sludge levels with an infrared detector

The infrared detector sludge level measurement method requires two people to go out on the aerated lagoon in a small boat to take measurements at various points. Sometimes both are Avizo Experts-Conseils technicians; other times, our technician is accompanied by a municipal employee.

When should the infrared detector method combined with lightweight disc validation be used?

The infrared detector method is mainly used on small and medium-sized lagoons. It provides reliable and accurate results. And since the technicians are already on the lagoon to take measurements, they can also collect sludge samples when needed.

Assessing the sludge level in an aerated lagoon using the infrared detector method with lightweight disc validation

1. Determine the location of the measurement points
2. Measure the total depth of the lagoon at the measurement point
3. Measure the depth of water above the sludge blanket
4. Calculate the sludge level
5. Validate the infrared detector measurements with the lightweight disc
6. Prepare the measurement report

1. Determine the location of the measurement points

A grid is established based on the size of the lagoon and in accordance with the recommendations of the Ministère des Affaires municipales et de l’Habitation (MAMH). For example, for a lagoon of less than 1,000 m², 10 measurement points are taken on the slopes of the lagoon and 10 elsewhere in the lagoon.

Lagoon bottom area	Minimum number of points on the lagoon bottom	Minimum number of points on the lagoon slopes
Less than 1,000 m²	10	10
1,000 m² to 2,000 m²	18	12
2,000 m² to 3,000 m²	32	16
3,000 m² to 4,000 m²	45	18
4,000 m² to 5,000 m²	50	20
More than 5,000 m²	The MAMH recommends using sonar	The MAMH recommends using sonar

Minimum number of measurement points recommended by the MAMH in the Guide pratique de mesure des boues dans les étangs d’épuration, 2013 edition

Technicians set up markers on the bank or stretch ropes across the lagoon. The distance between two consecutive measurement points must not exceed ten metres.

2. Measure the total depth of the lagoon at the measurement point

At each measurement point, a graduated pole is used to measure the total depth of the lagoon. The technicians lower the pole straight down to the bottom of the lagoon and record the total depth at that location.

3. Measure the depth of water above the sludge blanket

An infrared detector is used to measure the depth of water above the sludge blanket. When the infrared detector senses a change in density in its surroundings, it emits an audible signal.

The technicians slowly lower the infrared detector using a graduated rope. When they hear the characteristic sound and are confident that the sensor is indeed at the water-sludge interface, they record the depth of water above the sludge blanket.

Advantages and disadvantages of the infrared detector method

The infrared detector method requires two people on the water. Since the infrared sensor is an electronic device, it must be checked and properly maintained to make sure it works when needed.

However, when combined with lightweight disc validation, the infrared detector method provides very good results.

4. Calculate the sludge level

The sludge level at a given measurement point is obtained by subtracting the depth of water above the sludge blanket from the total depth of the lagoon. The operation is repeated at each measurement point on the grid.

5. Validate the infrared detector measurements with the lightweight disc

At least once per measurement line, the measurement taken with the infrared detector is validated using the lightweight disc method.

A lightweight disc fitted with a fine mesh is heavy enough to sink in water, but too light to sink into the sludge.

The technicians lower the lightweight disc into the lagoon. When it stops sinking, they know it has reached the sludge blanket. The distance between the surface and the sludge blanket is then recorded.

This measurement is compared with the one taken with the infrared detector at the same location to validate the infrared detector results.

Advantages and disadvantages of the lightweight disc method

The lightweight disc method is much more robust than the infrared sensor method. It uses equipment that requires very little maintenance: you only need to check that the disc is clean and straight.

The drawback of the lightweight disc is that it cannot be used to take measurements on slopes. Measurements on slopes can therefore only be taken with the infrared detector.

6. Final report: sludge level, wet sludge volume and dry sludge tonnage

The measurements taken by the technicians are compiled into a preliminary report by an Avizo Experts-Conseils engineer. This preliminary report is provided to the person in charge of the wastewater treatment facility.

If it is deemed useful to have the sludge samples collected during the measurement analyzed, a certified laboratory is hired to do so.

You will then receive a final report that may include, in addition to the sludge level for the lagoons measured, the theoretical calculation of the wet sludge volume in the lagoon, the dryness of the wet sludge and an estimate of the dry sludge tonnage that a future cleanout of the lagoon could generate.