Two highly practical standards for characterizing sewage sludge from wastewater treatment plants
With 5 liters per French person per day, wastewater treatment plants end up with millions of tons of sludge at the bottom of their tanks. To characterize their suitability for dewatering, with a view to recovery, professionals in the sector have developed a voluntary standard. Explanations.
Water management
Obtained after treating municipal or industrial wastewater in treatment plants, sewage sludge is a valuable resource, provided it is properly dewatered, compacted, and decontaminated. Operators must therefore characterize it according to its origin and its chemical, physical, and microbiological properties. They must also assess its suitability for concentration. Among these key parameters, one is particularly important: dryness, i.e. mass percentage of dry matter.
The dryness of sludge is determined after the drying and dewatering stages. Sludge is generally dewatered in belt or screw presses, plate filter presses, or centrifugal decanters. The higher the solids content of the sludge, the more suitable it is for one of the possible forms of recovery: agricultural spreading, composting, incineration alone or with other waste. Well-dehydrated sludge has a higher calorific value, greater agronomic value, causes less pollution, and reduces transport and storage costs.
To facilitate exchanges between equipment manufacturers and station operators, professionals felt the need to establish best practices in a voluntary standard. Published in mid-September 2018, standard NF T97-001, divided into parts 1 and 3, specifies the procedure for calculating a dryness limit value (part 1) and a reference value (part 3). As Pascal Ginisty, Scientific Director at IFTS (Institute for Filtration and Separation Techniques) and coordinator of the AFNOR working group Having drafted both texts, "the aim of this work was to define a common methodology and terminology, as well as a set of best practices to ensure the reliability of the operating procedures specific to players in the sector."
NF T97-001-1 and NF T97-001-3: limit dryness and reference dryness
Problem: sludge is difficult to characterize, including in terms of dryness, because the product varies greatly from one wastewater treatment plant to another, or from one day to another within the same plant. Of course, the origin of the effluent explains this variability, but so do the operating methods used at the plant, as not all operators use the same processes. Standard measurements were needed. The voluntary standard NF T97-001 does this job. For Pascal Ginisty, "the challenge was to define absolute values. For local authorities and industry players, these standards are more convenient and provide greater security. Thanks to them, there is consensus on the definition of the associated protocols and their validation in inter-laboratory tests on sludge from different sources."
According to standard NF T97-001-1, the limit dryness represents the potential for dewatering sludge that can theoretically be achieved by mechanical means. This measurement is useful for comparing the dewatering capacity of sludge from various sources, or conditioned in different ways, and allows the technical limits of this process to be assessed in advance. Similarly, according to standard NF T97001-3, the reference dryness acts as a key indicator for assessing the suitability of a sludge for mechanical dewatering, making it easy to compare different sludges.
Sewage sludge is a rich subject for voluntary standardization. Future work will focus on providing guidelines for recovering energy from these materials. "Better chemical conditioning facilitates treatment and recycling," concludes Pascal Ginisty.
