Construction projects such as bridges, underground parking garages, dams, and utility trenches are often constructed in, or near, the groundwater table. When excavations for these projects encounter groundwater, it must be removed in order to create dry working conditions. Sumps, deep wells, or well points are often installed to remove the water. Such water may be used onsite for dust control, or for other purposes as long as appropriate permits are acquired and approved. However, if water quantities exceed those needs, it must be stored and discharged.
Excess water is typically discharged to a sanitary sewer if possible, or “waters of the state” if necessary. Sanitary sewer discharges are pumped into a sewer line and treated at a local municipal sewage treatment facility. Discharges to waters of the state are pumped into stormwater inlets or other facilities that drain to a creek, bay, lake, or other water body that is regulated by the Regional Water Quality Control Boards (RWQCB). In either case, a discharge permit must be acquired from the appropriate regulatory agency prior to discharge.
Discharge permits typically set limits for common contaminants such as sediment, pH, volatile organic compounds (VOCs), hydrocarbons, and heavy metals. The degree to which water must be treated prior to discharge depends on the limits set forth by the regulatory agency. If a contractor encounters or produces water which exceeds those limits, a water treatment system must be implemented in order to comply with the associated regulations.
A contractor may unintentionally produce contaminated water by excavating in wet soils near the sump or well pumps, which produces suspended sediment that must be filtered or treated prior to discharge. A contractor may encounter contaminated water if they are excavating an area that was impacted by previous contamination sources, such as leaking underground fuel tanks. This document will deal primarily with sediment control because it is the most common contaminant encountered in construction site water.
Construction project specifications are often vague and provide little guidance for contractors attempting to design a water treatment system, so treatment system selection is typically performed by trained, experienced professionals who are familiar with local conditions and the regulatory environment. A careful study of site-specific soil types, construction processes, and water quality should be performed during the design process. Geotechnical reports, environmental site investigations, water sampling, and pilot scale testing can provide valuable information used in the design of a water treatment system on a construction site.
The contractor should consider the discharge options well in advance of starting the project. Permitting and associated regulatory challenges can take several months to resolve if appropriate planning is not performed. Determining where to discharge may be the most crucial decision a contractor can make regarding treatment system implementation.
Sanitary sewer system permits are often cheaper and more lenient than RWQCB permits. However, sanitary districts typically charge for every gallon discharged to their system, and flow rates are often restricted to 100 GPM or less. RWQCB permits cost more up front, but there is no “per gallon” fee, and there is no limit on flow rates. Determining where to discharge helps determine the type, size, and location of the treatment system.
When the contaminants of concern have been identified, the discharge location has been determined, and the discharge limits have been ascertained, a treatment system can be designed.
Water treatment systems may include settling tanks, sand filters, bag filters, chemical treatment, activated carbon, or other specialty media. The most common treatment systems are designed to remove sediment, and include tanks and filtration. Polymer flocculants are sometimes utilized if filtration and settling is not sufficient to meet discharge limits for sediment. Activated carbon can be implemented if VOCs or other organic contaminants are present. In the event that heavy metals or other hazardous substances (pesticides, bacteria, etc) are encountered, specialty media such as ion exchange resin may be required to meet discharge limits.
A groundwater treatment system can be effectively conceptualized as a train of modular components arranged “in series” (one after the other), like train cars. The importance of properly ordering the elements of the system should not be overlooked. For example, sediment should always be remediated prior to the use of activated carbon in order to minimize impaction of the carbon media. The most common system arrangement on construction sites is as follows:
- Water pumped from wells or sumps to the treatment system
- Sediment settling in tanks
- Sediment filtration with bag and/or sand filters
The system described above is designed for remediation of sediment only, which is the most common contaminant of concern to regulatory agencies. If additional contamination is encountered, a full scale system may be required, and may be arranged as follows:
- Water pumped from wells or sumps to the treatment system
- Inline polymer injection, mixing, and flocculation
- pH adjustment chemical injection with acid or base
- Sediment settling in tanks
- Sediment filtration with bag and/or sand filters
- Contamination remediation with activated carbon and/or specialty media
The arrangement described above is not set in stone, and many of the components listed above will not be used on most projects. Any system should be designed with options for the addition or removal of treatment modules as needed. A recirculation pipeline should also be installed to allow for additional treatment if the system does not produce acceptable results after one treatment cycle.
Temporary groundwater treatment systems are typically implemented without the aid of a pilot study that could prove the effectiveness of system design prior to implementation. It is important to note that geotechnical reports, environmental site assessments, and similar site investigations do not necessarily represent exact water quality conditions as they will be encountered onsite, and therefore cannot provide enough information to guarantee that the system will achieve compliance with discharge limits. The importance of selecting an experienced water treatment system provider should not be underestimated.
Creating a relationship of mutual trust with the appropriate regulatory agency is another important part of the process. Agency representatives can often provide useful information and guidance during the permit application process. Informal conversations with regulatory personnel are a good way to gain an understanding of a particular agency’s expectations prior to formal submittal of the permit application, thereby saving time that might otherwise be wasted on re-submittals.
A temporary groundwater treatment system consists of separate modules arranged in a specific order. They can be added or removed as needed, and are typically mounted on skids (platforms designed for structural support and handling) that can be moved with forklifts or other heavy equipment. A brief description of the most common elements of a treatment system are described below. Pumps, pipe, generators, flow meters, and other appurtenances are necessary to complete the system, but are not described in this document. These descriptions only apply to common applications, and do not account for atypical systems.
- Settling Tanks are typically constructed of steel, are rectangular in shape, and store 10,000-21,000 gallons of water. They range in size from about 25’-45’ long, 8’-12’ tall, and 7’-8.5’ wide. They are designed to allow maximum water travel and settling in the smallest footprint possible.
Tanks may be rated from approximately 50-300 Gallons Per Minute (GPM) per tank, depending on site specific conditions. Open-top and closed-top tanks are available, but open-top tanks are preferable.
- Sand Filters are constructed of steel, and consist of multiple “pods”, or vessels arranged “in parallel” (next to each other). 4-pod units are commonly used for groundwater treatment systems, and are mounted onto a single skid that can be moved with a forklift. The pods are connected together with manifold piping on the influent (top), effluent (bottom) and backflush (top) ports. Each skid may range in size from 15’-25’ long, 6’-7’ tall, and 3’-4.5’ wide. A typical 4-pod skid may range in flow rate from approximately 50-1000 GPM per unit.
Each pod is filled with a layered mixture of sand and gravel that is designed for remediation of sediment, and can last for several years if properly operated. Dirty water is pumped evenly into the top of the vessels, it travels downward through the media, and clean water is discharged out the bottom of the vessels. The dirt is trapped in the top layer of sand. Sand filters are self-cleaning, and utilize a backflush system to remove sediment buildup as needed. Media is typically vactored out and disposed at a landfill when the project is complete.
- Bag filters are constructed of steel vessels, each of which may contain one or several bag containers. Bag filter vessels are mounted on a single 4’x4’ skid that can be moved with a forklift. The bag media is typically constructed of polypropylene in an open-top, cylindrical shape, about 40” long and 8” diameter.
The bags are set into screened, metal containers to hold them in place. Each bag can typically process 50-150 GPM, which means that a 5-bag vessel may be able to process as much as 750 GPM, depending on sediment loading.
Bag filter media is a consumable which may last for several hours or several days. The media is usually disposed in an onsite trash container after it is spent. Replacement of the media takes about 5 minutes for a single bag filter.
- Activated carbon is typically used to remediate organic contaminants through the process of adsorption, whereby pollutant particles are trapped in the pores of the carbon granules. Carbon is particularly effective at removing VOCs, hydrocarbons, and sediment because the large pore structure creates an enormous surface area in which to trap contaminants. One pound of activated carbon has a surface area of approximately 100 acres.
Carbon media is typically delivered in steel vessels that are approximately 7’-10’ tall, 4’-6’ diameter, and mounted on a skid or trailer. Each vessel has an approximate flow rate of 70-500 GPM. The effective flow rate of activated carbon is very sensitive to fluctuating water quality. Sediment loading in excess of 10 Parts Per Million (PPM) Total Suspended Solids (TSS) can severely impact the effectiveness of the media in a relatively short period of time.
There are several different types and qualities of carbon. Virgin coconut shell activated carbon is typically used in groundwater treatment systems. However, carbon may be derived from bituminous coal, lignite, and other products.
Vessels are typically arranged in series, which is also known as “lead/lag”. A secondary set (lag) of vessels protects the system from discharge limit exceedances in the event the first set (lead) of vessels is compromised. Media is typically disposed at a landfill or regenerated after it is spent.
- Specialty media is an industry term describing a broad range of media that ranges from zeolite to resin to greensand. These media are used infrequently for remediation of a variety of contaminants that are particularly persistent and problematic. Heavy metals, pesticides, and pathogens are a few categories of pollutants that can be managed with specialty media.
The media is often installed and handled in similar fashion to activated carbon. However, it can vary greatly in particle density, shape, chemical composition, cost, operation, and availability. Each media has its own unique requirements for start-up, operation, regeneration, and disposal. Every product should be evaluated with assistance from the manufacturer’s representative.
The effectiveness of temporary groundwater treatment systems are often limited by several factors. Construction sites typically offer minimal footprints for water treatment equipment, which limits the ability of the contractor to install multiple tanks to enhance settling. Standard filtration media can remove sediment particles down to approximately one micron in size, but sub-micron particles will pass through filtration and enter the discharge flow. Filtration and settling alone may not be sufficient to remove sub-micron sediment particles, especially when space for tanks and filtration is limited.
However, these limitations can be overcome, and discharge compliance can be achieved with the judicious use of chemicals. Long chain polymers such as chitosan are highly effective in reducing turbidity levels by greater than 95% when used in conjunction with filtration systems. Chitosan’s effectiveness lies in its ability to bind small suspended soil particles into larger and heavier particles called ‘floc’. The cationic nature of chitosan molecules cause it to interact with the predominately anionic sediment particles in groundwater. As these opposite charges attract, the chitosan molecules can bind with numerous soil particles. This process of flocculation creates larger, heavier particles in solution, allowing them to settle via gravity or be removed by filtration.
Chitosan is a biodegradable, non-toxic flocculent. It occurs in nature as a biodegradation product of chitin, which is the structural material found in crustacean shells such as shrimp, crabs, and lobsters, and is also found in fungi cell walls and the exoskeletons of insects. Chitosan has been used in water treatment for more than three decades. It has the ability to absorb dissolved oil and grease from water, chelate (bond with) heavy metals, and flocculate suspended sediment. The California State Water Boards have approved its use on construction sites for storm water and groundwater treatment.
If polymers are used on a construction site for sediment remediation, certain protocols must be followed in order to comply with appropriate permits. Specific requirements for discharges of treated water from groundwater treatment systems to waters of the state are not always identified explicitly in groundwater discharge permits, but typically require adherence to a set of standards that were designed for operating Active Treatment Systems for stormwater remediation on construction sites. These standards can be found in the attachments section of National Pollution Discharge Elimination System (NPDES) permits for stormwater discharges that are maintained by the Regional Water Quality Control Boards. Protocols that are typically required include:
- Certified operator onsite at all times during chemical injection
- Automated recirculation in the event that water quality does not meet discharge limits.
- 15-minute automated recording of flow, pH and turbidity for influent and effluent water.
- Onsite residual polymer testing, and 3rd party verification by a certified laboratory.
- Regular jar testing, meter calibrations, chemical usage monitoring and recording.
- Discharge limits for turbidity are 10 NTU average, and 20 NTU for a single sample. Acceptable pH range is 6.5-8.5 standard units.
All chemical treatment systems are often referred to as active treatment systems, and are available in several different styles and sizes. Whichever system is selected should comply with appropriate permit requirements. They are typically constructed inside shipping containers, cargo trailers, or are mounted on flatbed trailers. The systems usually contain the following equipment in a single package:
- Flow, pH, and turbidity sensors which continuously monitor the effectiveness of the treatment.
- Datalogger for recording water quality information.
- Programmable Logic Controller (PLC) and Human Machine Interface (HMI) for controlling pumps, valves, and other system components.
- Automatically actuated valves that direct treated groundwater to recirculate or discharge based on the water quality readings of the sensors.
A summary of the approximate flow pattern and operation of a chemical treatment system follows. Groundwater is transferred from wells to a storage tank, at which time liquid chitosan polymer is introduced to the water using chemical injection pumps and a static mixer. The polymer flocculates suspended solids into larger, heavier particles called ‘floc’, the majority of which will settle out in the tank. A pump will then transfer the treated water from the storage tank through a filtration stage. A second injection of chitosan may be used to aid in the removal of sediment by the sand filter. The sand filters are equipped with automatic backflush systems, which regularly remove trapped sediment in order to maintain the hydraulic capacity of the filter. This feature allows the treatment system to operate on a continuous flow-through basis. Bag filters provide additional filtration of remaining fine sediment particles. If water quality parameters are achieved, the discharge valves will open and send water to the discharge point. If further treatment is necessary, the recirculation valve will open, sending water back to the tanks for further treatment. Influent and effluent water quality are recorded automatically by the datalogger, and manually by the operator.
Additional modules may be added to a chemical treatment system, such as pH adjustment or air injection. Many systems have unique permit requirements that are specific to a particular project, and may require monitoring and treatment for water quality parameters such as dissolved oxygen. The most common issue requiring additional treatment is high pH resulting from concrete pours. Freshly poured concrete particles dissolve and create high pH when they encounter water. This problem can be mitigated in the short term through the manual introduction of dry ice into tanks. If the problem persists, an acid injection module can be added to the system for continuous treatment. Dry ice and acid can be extremely hazardous, and should be handled accordingly. Consult the associated Safety Data Sheet (SDS) for handling and Personal Protective Equipment (PPE) recommendations. Such information should be available from the product seller or manufacturer’s representative.
Contractors are typically responsible for designing, implementing, and monitoring water treatment systems on their projects. Given the minimal information that is typically provided by project owners, this task presents a serious challenge. The operation of the system itself may seem simple compared with the planning necessary during the pre-construction phase. However, these challenges can be met through careful and timely preparation. The most crucial questions to be answered are:
- Where to discharge? Sanitary sewer? Waters of the state?
- Which agency is responsible for regulating the discharge? How long does it take to acquire a permit from that agency? What is the cost?
- What is the estimated flow rate? Will flow rate decrease over time?
- What is the expected water quality, and discharge limits?
These questions should be answered carefully so that appropriate options can be compared, and the project moves forward in the right direction. System design, permit application, and implementation of a water treatment system can take 6 months or more. Pre-construction planning will minimize project delays associated with installation of a water treatment system, and create positive outcomes for all parties.