Why and how to measure pond sludge?

Measurement and Sampling of Sludge Produced in Wastewater Treatment Ponds: Why and How?

Why and how should sludge produced in wastewater treatment ponds be measured and sampled? Operators of wastewater treatment plants must regularly ensure that sludge levels in aerated ponds do not exceed the recommended limit to maintain optimal performance, comply with regulations, and adhere to the initial design parameters for equipment performance.

A quick glance at a map of Quebec reveals the technological choices made by designers in proposing treatment systems for most cities and municipalities across the province.

The Natural Principle of Lagooning

Lagooning is a process based on the natural purification phenomenon that occurs in stagnant bodies of water like lakes or ponds, leveraging aquatic vegetation as a purifying agent for polluted water. The plants thriving in these environments serve as support for aerobic bacteria, which accelerate the degradation of organic matter in the water. Additionally, as water slowly passes through these natural filters under UV exposure over a large surface area, nitrogen- or phosphorus-based elements are absorbed by the plants.

By replicating these ecosystemic conditions through the installation of artificial ponds, their natural filtering power becomes a valuable resource in water treatment processes. As a result, lagooning processes discharge water that is purified of many undesirable impurities before being released into natural environments. Due to its operational simplicity and the availability of extensive space, this solution has been adopted by around 700 treatment plants.

Aerated and Non-Aerated Ponds

Aerated and non-aerated ponds represent the most widespread technology in municipal water treatment. For instance, in 2020, aerated pond systems accounted for nearly 70% of wastewater treatment plants in operation, in Quebec.

Regardless of the type, municipal ponds must be properly maintained to ensure and optimize their efficiency. Despite its high autonomy, the lagooning purification process generates sediments that accumulate at the bottom of ponds and must be periodically removed. Why? Because excess sludge accumulation beyond a certain threshold reduces the water retention time in basins, lowering the efficiency of the treatment process. Additionally, adhering to this threshold prevents resuspension of materials deposited at the pond’s bottom and ensures the structural integrity of the installations. Consequently, if sludge levels approach 15% of the pond’s theoretical total volume, emptying is recommended to maintain proper performance.

In Quebec, municipal wastewater treatment infrastructures are governed by the Regulation Respecting Municipal Wastewater Treatment Works (ROMAEU) and regulations from the Ministry of Municipal Affairs and Housing (MAMH). Municipalities are required to regularly report the condition of their installations to the relevant authorities. Among the monitoring requirements, verifying sludge occupancy as a percentage of total pond volume (%v/v) is no exception and must be performed at least once every three years under favorable conditions.

Measuring Sludge Volume in Ponds

Measuring sludge volume in ponds is a specialized service offered by Avizo, enabling cities and municipalities to easily meet their sludge monitoring objectives through various methods. By measuring sludge height at multiple mapped points on the pond’s surface, the accumulated sludge volume can be calculated based on the pond’s geometry. Whether rectangular (standard) or occasionally circular, the approach remains the same; the volume calculation is simply adapted to the measured heights.

The MAMH (formerly MAMROT) has developed a reference document, the Practical Guide for Measuring Sludge in Wastewater Treatment Ponds (MAMH, 2010), which remains the primary reference in Quebec for sludge measurement. The techniques currently offered by Avizo include the following:

Étang aéré — Rectangular pond (standard)

Étang circulaire — Rectangular pond (standard)

The Light Disk Method

This conventional method employs a light disk or sludge gauge. The light disk is a horizontally positioned disk lowered into the water until it rests on the sludge surface; a measuring tape attached to the disk allows reading the water depth it traveled. This method is cost-effective and precise but has slower execution speeds. It is often used as a redundancy measure to validate results obtained through other methods.

The Sludge Judge Method

The sludge judge involves using a long, transparent cylinder to directly measure sludge thickness and the supernatant above it. Due to the equipment’s limited maneuverability in water bodies, this method is better suited for smaller non-aerated ponds or shallow lagoons, as well as sedimentation basins or septic tanks. It is also ideal for basins with irregular bottoms, such as natural ponds.

The Photoelectric Cell Probe Method (Infrared Detector)

Similar to the light disk, this indirect method employs a probe that uses infrared waves to detect the sludge surface. By progressively submerging the probe, it emits infrared waves and distinguishes signals based on the medium encountered. This method accelerates measurement collection and offers a good precision-to-cost ratio.

Sonar Measurement Method

Sonar is widely used in Quebec and directly determines sludge depth using a high-frequency signal reflected by the sludge layer. As the sonar attached to a boat moves across the pond surface, sludge depth is calculated continuously. For ponds larger than 5,000 m², sonar is highly efficient due to its time-saving capabilities and provides detailed results.

Sludge Removal

When sludge removal is necessary, operators must manage large quantities of material for disposal or valorization. Sludge quality must be analyzed every three years and in the year preceding removal to determine management options. Generally, sludge sampling involves at least five samples distributed across the pond and homogenized into one composite sample for laboratory analysis. Agricultural valorization as fertilizing residual material (FRM) is a cost-effective and environmentally friendly option. This requires compliance with the Guide for Recycling Fertilizing Residuals regarding sampling methods and analysis parameters. Otherwise, sludge must be sent to a sanitary landfill, adhering to the Regulation Respecting Hazardous Materials (RHM) and the Landfill and Incineration Regulation. While less restrictive, this option is more costly.

Avizo offers sludge measurement services, standard and accredited sampling (DR-12-MRF-02), and bid documentation preparation for sludge removal and disposal. Contact our professionals today for a quote.

Measurement and Sampling of Sludge Produced in Wastewater Treatment Ponds: Why and How?

Measurement and Sampling of Sludge Produced in Wastewater Treatment Ponds: Why and How?

The Natural Principle of Lagooning

Aerated and Non-Aerated Ponds

Measuring Sludge Volume in Ponds

The Light Disk Method

The Sludge Judge Method

The Photoelectric Cell Probe Method (Infrared Detector)

Sonar Measurement Method

Sludge Removal

References

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