The Process of Water Reclamation

 

Water is an important resource no matter where you are, but it is doubly precious in the desert. Each day in Las Vegas alone, everyday life generates 170 million gallons of wastewater. In order to maintain a city of Las Vegas' size with the limited resources available, the city must take steps to reclaim wastewater at every possible opportunity. The purpose of the Clark County Water Reclamation District is to reclaim as much wastewater as possible, and a single facility on Flamingo Road is responsible for cleaning 96 million gallons of wastewater per day and making it safe to use once more.

 

The Cycle of Wastewater

 

The Beginning

It all starts innocently enough. You take a shower, or wash your hands, or flush the toilet. The water used for these processes flows down the drain and into the sewer system, where it joins all the other sewage in a journey through miles of pipe and eventually reaches a Wastewater Treatment Plant. At this plant, wastewater is subjected to four levels of filtering designed to remove large objects, smaller objects, grit and dirt, sludge, oil, and bacteria. After around 8.5 hours in transit, this cleaned water (effluent) is not safe for drinking, but is clean enough to be pumped back into the Las Vegas Wash and Lake Mead. The purpose of this document is to describe the process of turning sewage back into water used at the Clark County Water Reclamation District.

 

Figure 1: A process diagram of the reclamation process used at CCWRD Flamingo.

 

 

 

Wastewater Treatment Plant

A Wastewater Treatment Plant is exactly what it sounds like - a facility designed to turn sewage back into useful water once more. Every major city has at least one Wastewater Treatment Plant; they are as necessary as police stations and hospitals, though not nearly as visible. At the plant, sewage undergoes several processes to turn it back into useful water.

 

This video shows the environment deep inside a wastewater treatment plant.

 

Pre-Treatment

The “Pre-Treatment” stage involves the removal of foreign bodies from water. Foreign bodies in this instance refers to solid objects and particles like tree branches, twigs, leaves, gravel, old rags, and any object that is flushed down the toilet or dropped down a drain.

 

• Straining. Straining is the act of removing large solids, like trash or foreign objects, from the sewage. These objects could cause damage to the delicate filter apparatus down the line, so removal at this stage is important. These removed solids are compacted to remove any remaining water and sent to the landfill.
• Grit Removal. The straining mechanism used is not perfect, and small solids like sand and coffee grounds can still get through. During the grit removal process, sewage is aerated to lower its density, allowing heavier items to fall out of solution. These solids are spun to remove any traces of water and then sent to the landfill.

 

By this point, many of the largest solid particles have been removed from the water, but there is still plenty of work to be done.

 

Primary Treatment

After pre-treatment, what is left is still sewage. There is plenty of organic material remaining, and the sewage resembles a brown, foul-smelling soup more than water. Primary Treatment is intended to separate water from dissolved and organic solids, such as human waste and other contaminants. This is accomplished through the use of several processes.

 

• Sedimentation. To encourage the separation of water from the solids suspended or dissolved in it, sewage is allowed to move slowly for a period of several hours, during which sedimentation occurs. Sedimentation is a process by which contaminants are allowed to settle to the bottom (in the case of human waste and dirt) or float to the top (in the case of grease, fats, and oils).
• Chemically-Induced Separation. During primary treatment, chemicals are added which bind, or attach themselves, to contaminants. These attached chemicals make the contaminants heavier, causing them to fall out of solution.
• Skimming. Once sedimentation and separation have occurred, a pair of skimmer arms move through the sewage. The arm at the top removes grease and oil, siphoning it off to be safely disposed of. The arm at the bottom removes sludge, which is set aside for further treatment.

 

 

Secondary Treatment

Primary treatment removes approximately 65% of contaminants in wastewater, but this product is still considered sewage and requires further treatment to be made useful. Secondary Treatment uses many of the same techniques as primary treatment, though it has a few tricks of its own, as well.

 

Bacterial Inoculation. To further reduce the amount of organic material, specific bacteria are introduced into the sewage and then given time to grow. These bacteria feed on organic waste and are therefore helpful to the treatment process. Activated Sludge. Once these bacteria feed and multiply, they become what is known as activated sludge, a sort of bacterial mass that feeds on contaminants. Most of the time, this sludge is created by reintroducing some of the sludge used in a previous batch and then allowing the bacteria to grow. This sludge is effectively self-sustaining, provided it has oxygen, and can be used many times over. Sedimentation. In the final stages of secondary treatment, the sludge is allowed to settle out of the liquid, and gets recycled into the next batch of sewage using a skimmer system very similar to the one used in the primary clarifier. Filtering. The sewage is then passed through a series of increasingly fine filters, removing even more sludge and particulate matter.

What happens to the sludge? Some of the activated sludge is recycled into another batch. However, since each batch generates most of its own sludge, there is plenty that must be disposed of. This is pumped to a separate section of the treatment facility, where it is compressed mechanically to release any remaining water (which is then sent back to Primary Treatment) and then sent by truck to a landfill. Each day, more than 300 tons of sludge are generated through this process.

 

Tertiary Treatment

Secondary treatment removes approximately 90% of total remaining pollutants. Combined with primary treatment, the wastewater only contains about 4% of the contaminants it originally held. Still, the water is home to billions of potentially harmful bacteria and other organisms, so further work is required.

 

Natural Media Filtration. The water is pumped into a natural media filter, consisting of layers of large gravel, small gravel, sand, and coal. These natural filters remove any remaining tiny particulate solids from the water, as well as any remaining chemical additives from earlier stages in the process. Ultraviolet Disinfection. Water leaving the media filters is then subjected to intense ultra-violet light. This light does not just kill bacteria, it disrupts their genetic code such that they cannot reproduce. This ensures that very few bacteria make it through the UV filtering alive. Outflow. The resultant water is finally clean enough to be returned to Las Vegas Wash and, eventually, Lake Mead. Some water is not sent to Lake Mead and is instead treated with Sodium Hypochlorite (a bleaching agent) and used for irrigation or industrial purposes.

Odor Control. During the processing of wastewater, many gases are released. These gases are created as a result of the organic material (mostly human waste) decomposing. Gases such as methane and hydrogen sulfide are not only foul-smelling, but also dangerous. Bio-filters, large tanks filled with colonies of odor-eating bacteria such as thiobacillus, eat the odor-causing particles in the air and release only clean air to the outside world.

 

The water reaching the Las Vegas Wash bears almost no resemblance to the foul sludge that entered the treatment plant that morning. It looks and smells like clean water, and indeed it is clean enough to water our lawns and crops. This water is not safe to drink, however. Drinking water undergoes even more purification at the Las Vegas Valley Water District, where it is sedimented, filtered, filtered again, subjected to ozone treatments, and chlorinated before being pumped all over the Las Vegas Valley. 

 

These methods are complex and take time to work properly, but the end result is a very high rate of reclamation for Las Vegas. This process is what allows life to continue as normal in the desert, where water is more precious than most people know.

 

Source: Facts and figures obtained from Clark County Water Reclamation District website, http://www.cleanwaterteam.com. Accessed 2/24/10.