Coordination with permitting and regulatory entities, such as the U.S. Environmental Protection Agency (EPA) is a must before performing water treatment plant upgrades. The EPA works with federal, state and regulatory partners to protect human health and the environment. The Clean Water Act governs certain aspects of water pollution from wastewater management to pretreatment and stormwater pollution.
Membrane Filtration and Softening
One common water treatment plant upgrade includes adding membrane filtration and softening to an existing facility. Membrane softening is often considered to meet customer desires, eliminate the need for in-home softeners, and reduce total dissolved solids being sent to water resource recovery facilities (WRRFs). However, before implementing these upgrades, aspects of a membrane softening addition may require permitting and coordination that include, but are not limited to:
- Installation: A permit to install (PTI) new equipment or components will be needed from the EPA.
- Capacity change: Authorization from the EPA is required to change the capacity of an existing plant or the rated capacity of certain treatment components. This change is due to the increased raw water demand to produce the same amount of finished water as the existing plant because of the membrane waste stream flow. High rate filtration studies may need to be completed if loading rates exceed standards.
- Change in water quality: As a result of a change in water quality into the distribution system, the team should perform a corrosion control study.
- Building, electrical and plumbing: Building permits will need to be acquired, based on the local jurisdiction. In addition, associated electrical and plumbing permits will be required.
- Waste stream discharge: If a project includes the discharging of a membrane waste stream into surface waters or blended with a WRRF effluent stream, a National Pollutant Discharge Elimination System (NPDES) permit must be obtained or modified.
Disposal of Concentrate
It is typical for membrane filtration to operate at or near an 80% recovery rate, meaning that 20% of the water fed into the membranes will be discharged as a waste stream. This waste stream will also be constituent-heavy as it will contain the constituents removed from the filtered water, known as permeate. Discharging of this waste stream is a crucial aspect of a membrane filtration plant. Options to release this waste stream include discharging to a sanitary sewer network, surface water discharge and blending with a nearby WRRF effluent.
Discharging directly to the sanitary sewer is likely an undesired option due to capacity limitations at the WRRF or collection system. Discharging to a nearby surface water, such as a stream, would require an NPDES permit. The high constituent strength of the waste stream may make this challenging to achieve. Additionally, a mass loading balance study of the receiving stream and the waste stream will likely be required. If an NPDES permit will not be granted for the waste stream, it is possible that a modified NPDES permit of a nearby WRRF could be achieved instead.
This option includes blending the water plant waste stream with the effluent of a nearby WRRF and modifying the existing NPDES permit to accommodate for this. The degradation of the receiving stream may be reduced by receiving this blended flow because the heavily concentrated water plant waste stream would be diluted by the wastewater plant effluent.
Coordination with the EPA on receiving an NPDES permit or modified existing NPDES permit should occur before the design of a membrane filtration addition, or at least in the very early stages of design to facilitate permitting and understand regulatory expectations before unnecessarily getting ahead with design. This coordination limits rework, saves time and limits frustrations associated with a critical component of membrane softening projects.
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