Sludge is a byproduct of many types of wastewater treatment systems and therefore an important parameter to sample and test. It can contain MRSL substances (that have the tendency to settle down/ partition into sludge during the wastewater treatment processes), metals from dyes, pigments, cotton, piping systems, and pathogens that can harm the environment and human health. Almost all facilities generating sludge are currently required by law or permits to sample and test the sludge to determine if it classifies as hazardous or non-hazardous before disposal. However, these global law/permit requirements do not cover all parameters and limits set by ZDHC. By including sludge in the scope of the guidelines, ZDHC can help to ensure zero discharge of hazardous chemicals from the key pathways of wastewater treatment systems.
What are the pathways of sludge disposal?
The ZDHC Sludge Reference Document and ZDHC Wastewater Guidelines V2.1 lists the following 7 pathways:
Pathway A - Offsite incineration at >1000 °C, not owned or operated by the supplier.
Pathway B - Landfill with significant control measures, that control the leachate and gas produced from the stored sludge in the landfill.
Pathway C - Building products processed at >1000 °C, which includes manufacturing concrete, concrete aggregates, ceramics, brick, mortar, stucco, grout, and more.
Pathway D - Landfill with limited control measures, where permeability, leachate, and gas control are less restrictive than in Pathway B.
Pathway E - Offsite incineration and building products processed at <1000 °C, not owned or operated by the supplier.
Pathway F - Landfills with no control measures (these are mainly dump piles and holes with no lining or packing to limit waste exposure to the ground and/or groundwater)
Pathway G - Land applications include pastures, forests, reclamation sites, agricultural land, public parks, street median strips, golf courses, lawns, and home gardens. The sludge must meet a quality determined to benefit the area soil and harmless to public health and the environment for such a disposal pathway.
How does a supplier do the sludge testing as per the ZDHC Wastewater Guidelines V2.1?
Suppliers should ensure that the testing of sludge is completed as detailed under Table 4A-4D in the ZDHC Wastewater Guidelines V2.1 document. If there is any non-conformity to the limits applicable to the supplier’s disposal pathway, a Root Cause Analysis (RCA) should be done and a Corrective Action Plan (CAP) should be developed by the supplier. The methodology is as follows for different sludge disposal pathways:
How should a facility declare its sludge disposal pathway?
For a facility, there could be more than one method of sludge disposal. In that case, the lab needs to conduct the testing only on the method used to dispose of the majority of the sludge rather than conduct testing on all methods.
ZDHC does not intend the lab sampler to assess or confirm the major disposal pathway adopted by the supplier. Instead, the facility must declare its major sludge disposal pathway in the Supplier Profile in its ZDHC Gateway account. The ZDHC lab sampler only needs to confirm (before travelling to the facility for sample collection) that the facility has made this declaration, and then note what the pathway is (from A to G) for purposes of interpretation of the sample results by the lab. For further information on the sludge disposal pathways, please refer to the ZDHC Sludge Reference Document V1.0 (https://downloads.roadmaptozero.com/output/Sludge-Reference-Document).
What is the “disposal pathway” that a laboratory should mark in the ZDHC Gateway Supplier Profile and conduct sludge testing if they are unable to find any conclusive evidence of any disposal pathway(s) either from the sampling facility or the third-party sludge disposer?
Where there is no evidence available for the declared pathway. The lab should test the sludge for parameters of “disposal pathway F” which has the strictest threshold limits out of all sludge disposal pathways.
How do you tell if a ClearStream report is compliant on sludge?
If all of the required parameters are met, it is indicated by a “green tick” in the ClearStream report. If any of the parameters are not within the required limits, this is indicated by a “red cross” in the ClearStream report. This means that the sludge does not meet the required disposal parameters and the supplier should complete an RCA (Root Cause Analysis) and CPA (Corrective Action Plan).
What are the best practices for sludge treatment and storage?
It is the responsibility of the supplier to treat all the sludge generated to reduce its volume and weight through appropriate thickening, digestion, conditioning, and drying (refer pages 16-17 of ZDHC Sludge Reference Document V1.0 for more details) (https://downloads.roadmaptozero.com/output/Sludge-Reference-Document)
Sludge should be stored in an adequate storage area, with at least 50% higher capacity than the average annual sludge generation quantity. The storage area should be designed to withstand the load of material stored and must be well-ventilated. The flooring of the sludge storage area should be impermeable and inert. The sludge storage area should be enclosed or covered and protected from the weather (precipitation or wind). Secondary containment should be provided for the sludge storage area through the use of barriers to contain the sludge inside the storage area. Dry sludge should be managed so that it is not blown from the area to the surrounding environment.
As per the ZDHC Sludge Reference Documents V1.0, a supplier must maintain a sludge document at his facility that details the following (refer to page 19 of the ZDHC Sludge Reference Document V1.0 for more details) (https://downloads.roadmaptozero.com/output/Sludge-Reference-Document)
Local regulatory norms and permits for sludge storage, transport, and disposal
Mass balance of all sludge generated and disposed
All sludge disposal pathways adopted by the facility (out of the 7 pathways listed in the ZDHC Sludge Reference document), noting the major pathway (in case of more than one pathway is adopted). The sludge disposal details should also be completed within the Supplier Profile in the Gateway account.
If the facility is disposing sludge to authorised third-party contractors, details on the disposal pathway used by this authorised contractor should be obtained by the supplier and documented. (Please refer to pages 20- 25 of the ZDHC Sludge Reference Document V1.0 (https://downloads.roadmaptozero.com/output/Sludge-Reference-Document) to understand the information details needed to be collected for each of the 7 disposal pathways)
Test results for the sludge under local laws and under the ZDHC Wastewater Guideline V2.1
Waste manifest or similar local transportation document required for each shipment, including;
Mass/volume of sludge shipped
Disposal or processing facility that the sludge was shipped to
Signature of personnel at the disposal or processing facility certifying acceptance of the sludge.
Copy of the authorised third-party waste contractor license or permit (in case applicable) and/or agreement to demonstrate that the contractor is properly permitted and licensed to handle and process the sludge.
What is the total threshold value of cyanide in the sludge?
Note: Cyanide is an anion, not a metal. Therefore, there is no total threshold value for cyanide.
TCLP (Toxicity Characteristic Leaching Procedure) analysis needs to be done only for metals in Table 4B if they exceed the respective total threshold values.
Cyanide is listed in Table 4D of the ZDHC Wastewater Guidelines V2.1 with each value that needs to be tested in sludge based on the sludge disposal pathway (A-G).
See below for reference
Referring to the above diagram, “point 1” is general testing based on the major sludge disposal pathway [out of the seven pathways (A-G) mentioned in the ZDHC Wastewater Guidelines V2.1]. There, ZDHC MRSL parameters, heavy metals, and anions (cyanide) should be tested accordingly.
For on-site incineration/energy recovery, “point 2”, ZDHC MRSL parameters and anions (cyanide) should be tested in the composite sludge before the incineration/energy recovery process.
“Point 3”, heavy metals should be tested in the ash after the incineration/energy recovery process based on the major disposal pathway of the ash [out of the seven pathways (A-G) mentioned in the ZDHC Wastewater Guidelines V2.1].
What is the update to sludge testing?
The following updates have been made;
Sampling point 1: Sludge that is not incinerated on site and disposed of to the environment.
All tests from Table 4A-C should be done and reported for conformance to the supplier’s major disposal pathway.
Sampling point 2: Sludge that is incinerated on site.
Only non-metal MRSL substances and cyanide are to be tested. This is basically to determine whether these substances are present or not. If they are, then a root cause analysis is required.
Sampling point 2: Ash generated from incineration
Should be tested for metals as per Table 4A (total metal) and 4B (leachate) as per major disposal pathway.
In case all the sludge is incinerated on site
There is no need for sampling and testing as per point 1 in the diagram. Only testing as per point no 2 and 3 has to be done.