Water Reycle Systems

What Water Sources can be Recycled?

Lets look at some definitions of water types for the purpose of recycling. 

1. Utility Fresh Water:  This is purified water that you pay for, usually from a pipe with a meter.  Or it could be a private well with costs for pumping and treatment.
2. Utility Recycled Water: This is treated water from a municipal wastewater treatment plant that can be used in irrigation systems.
3. Rainwater: Water that falls on your property.  It is not necessarily free because it needs to be collected and lightly treated for dirt, trash, or oil that it may pickup from surfaces.
4. Greywater:  Water that you paid for once and used lightly in showers, sinks, and laundry (no toilets or kitchens).  It can be treated and re-used typically for sub-surface irrigation.
5. Washwater:  Water used in car, truck, and equipment wash facilities.  It can be treated and re-used in the same process.
6. Blackwater:  Water that you paid for and used more heavily, including toilets and kitchens.  This is expensive to treat onsite and is typically discharge to centralized wastewater treatment, or off-grid treatment systems.

We will focus on Rainwater, Greywater, and Washwater as the typical facility level water sources for recycling efforts.

How is Recycled Water Used?

Now that we have sources, what is the end-game?  How is recycled water used to advantage? Here are the most common applications:

1. Landscape Irrigation.  Harvested Rainwater lightly treated and used as landscape irrigation water. Some greywater may also be treated and used as irrigation water, in some cases limited to sub-surface irrigation.
2. Toilet Flush Water.  Larger building systems may use harvested rainwater and treated greywater as toilet flush systems.  These systems require independent distribution piping, but can replace a significant use of water in buildings.
3. Cooling Tower Water.  Cooling towers evaporate water to cool buildings.  Data centers and hospitals may have very large systems for these purposes.  Recycled water can be used for cooling tower makeup water, with special attention to water quality required for the application.
4. Equipment Washwater.  Equipment wash water can be cleaned for re-use at the point of the wash operation saving significant costs and avoiding environmental discharge problems.
5. Groundwater Aquifer Recharge.  Large wastewater operations may recycle treated water by flowing water to recharge groundwater aquifers.  This can be especially important where freshwater aquifers adjoin ocean environments and aquifer depletion can be a significant problem.

What is the Treatment Flow?

Now lets look at what happens between the Source and Point-of-Use for Recycled Water.

Water Collection 

Rainwater Harvesting.  Rainwater harvesting consists of collecting rainwater from rooftops, pavements, or landscaped areas The water naturally flows by gravity across flat surfaces to channels, sumps, and in-ground vaults.

Greywater Collection.  Greywater collection will be from standard sinks, showers, and wash stations.  Importantly these flows will be separated from toilet and kitchen and other flows directly to sanitary sewer systems.

Washwater Collection.  Washwater usually starts with applying water to clean vehicles and equipment using a pressure washer.  This is usually done on an impervious concrete pavement, or a specialized wash stand.  Walls or curtains may be installed on the surface perimeter to account for spray.  Wash areas will include channels or sumps to collect water, and these may have an important function in settling gross dirt or sludge from the wash operation.

Water Conveyance 

Water flows firstly and without effort by gravity across pavements into sumps and through piping systems. The main concern is designing the systems for the maximum expected flow, and eliminating trash and dirt to the maximum extent possible.  Small inlet details such as filter fabrics and small separators may be used to optimize system performance.

In Greywater piping systems standard sewer flow design slopes should be used to assure scour of solids in the bottom of the pipe.

At the point of final collection, water may gravity flow into a storage tank or vault or possibly just a small sump.  Then a pump with automatic control is likely used to push the water to treatment.  The pump system need to be designed for required flow capacities with associated controls.  Commonly dual pumps are used for reliable operation.

Primary Treatment 

It is possible that collected water is very clean and can be immediately sent to pint of use.  More likely it has accumulated some trash, dirt, oil, or other contaminant from surfaces during its journey.

The most simple treatment of collected water is gravity separation of trash (likely floating), dirt (likely sinking), or oil (likely floating).  Water has a specific gravity of 1.0, so these contaminants will sink or float depending on their specific gravity.  For example approximate specific gravities are Steel 7.8, Oil 0.85, and Dirt 2.7.

The issue is that the separation takes time as it can progress slowly.  If you have a long slow moving body of water, then separation happens naturally.  However this may be a large and expensive structure.  So most oil water separation systems includes baffles of some type to require only a short journey to the collection of water or dirt on a surface.  Some separation systems may be designed for centrifugal separation, directing water to flow across the walls of a cylindrical structure to a capture surface.

In some processes, chemicals may be added to the water flow to encourage flocculation or coagulation of dirt and oils into large particles which will separate from the water more quickly.

Secondary Treatment

For rainwater harvesting applications with relatively clean collection, the primary treatment noted above may be all that is required for the recycle use. 

Granular Media Treatment: Other inlet water flows or use applications may require additional treatment, commonly Granular Media Filtration.  This may be a gravity flow sand filters through horizontal structures, or pump fed pressurized systems with cylindrical vessels.  Granular media may be simple sand, activated carbon, or one of many media choices between. 

Purification: After primary and secondary treatment, additional treatment may be required to improve color, clarity, odor, and other measures.  Disinfection may be required where pathogens may be present.  As you would expect these treatment methods are all more costly than the primary and secondary methods, so it is important to analyze the water quality requirements to design these systems to be effective and cost-effective.

• Membrane Filtration:  Pumping water under higher pressures through cylinders of membrane so fine as to separate molecules of different sizes and characteristics.  
• Ozone Treatment:  System to generate ozone and treat water flows.  Ozone is a strong oxidizing agent, 50% stronger than Chlorine.
• UltraViolet (UV) Light Treatment:  Systems exposing water flows to ultraviolet light for dis-infection.  UV has the advantage that no chemicals are added to the water being treated and that no disinfection by-products are formed
• Chemical Disinfection:  Chlorine and like chemicals injected in water flows to provide dis-infection.
• pH Adjustment:  Usually used upstream in primary and secondary treatment where pH adjustment leads to better performance of processes.
• Bio-Reactor / Membrane Bio-Reactor (MBR):  A simple bio-reactor may be utilized to remove organics from water by adding bio-media and agitating with air flows.  Membrane Bio-Reactors allow for very highly effective organics treatment and removal from water streams

Water Storage Tank Systems

Water storage tanks may be used at the beginning, end or intermediate points in the water re-cycle systems.

Storage tanks for water recycling are commonly HDPE plastic, Fiberglass, Coated Steel, or Concrete.  Underground tanks would be horizontal cylindrical fiberglass or coated steel.  Underground concrete tank structures would be rectangular, or in some cases vertical cylindrical. 

Aboveground vertical cylindrical tanks made from HDPE are often the least costly option and are widely used.  Tanks can be insulated and heated for cold weather operation.

Rectangular HDPE, Fiberglass, or Metal tanks are used aboveground to meet space limitations or provide a required water flow path.

Tank level monitoring and control is usually provided and integrated into a Primary process control system.  Level transmitter may be top mounted ultrasonic type or submersible pressure type.  Point level sensors for high and low level are available in a range of base technologies.

Water Pump Systems for Recycling

Water pumps used in recycling are typically centrifugal pumps.  These may be horizontal mount, vertical mount, or submersible.  Special attention must be paid to pump selection where pump performance could be affected by high solids content, or corrosive environments.

Specialized small pumps may also be used in the water recycling process.  Metering pumps are small precision flow pumps for introducing treatment chemicals.

Integrated Control and Monitoring for Water Recycling

Water Recycling controls is typically done with a PLC based control panel, Touch Screen User Interface, and capability for remote monitoring and control.  Controls must be designed with digital and analog input / output capacity to meet the application.

Pump and Motor Control are the primary functions of the control system.  Pump motors are the largest requirement for power and pump operation should be examined to optimize efficiency and minimize power consumption.

Controls Systems are often mounted in stainless steel weatherproof enclosures with temperature control.

Control system components besides the controller will include: motorized valves, motor starters of VFDs, tank level sensors, temperature sensors, pH and other water quality sensors.

Startup, Performance Testing and Integrated Commissioning for Recycled Water Systems

Water Recycling systems are all somewhat unique.  They are built form standard components and sub-systems arranged in a system designed for a specific application.  For this reason it is important to have technical confirmation that the systems performs as required.

Startup is a procedure that an individual pump, or tank, or component works safely and as intended.  System testing goes well beyond startup.

Performance testing begins with a review of the intended Sequence of Operation.  Then a step by step detailed test procedure is designed to check the performance of each component and subsystem – including failure scenarios, warnings, and alarms.

Finally and integrated testing procedure is designed and performed to achieve customer acceptance.