DESCCO Water Treatment Construction: Frequently Asked Questions

The industry of water and wastewater treatment plant construction can be a bit challenging to navigate. Courtesy of DESCCO Water, your most trusted water and wastewater treatment plant contractors, here are a few clear and concise answers to some of the most frequently asked questions in the world of wastewater treatment, water treatment plant design, and wastewater treatment plant construction.

Typically, there are two main types of wastewater sources: domestic sewage, and non-sewage. Domestic sewage includes all wastewater generated from residential communities, public restrooms, hotels, golf courses, restaurants, schools, hospitals, and other health centers. Non-sewage includes all industrial wastewater, storm water, runoff, and water from swimming pools, car garages, and cleaning centers.

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Wastewater treatment is incredibly essential to the protection of human health, especially against pathogens and chemicals. In addition, proper design and construction of wastewater treatment plants protect the environment by removing harmful metals and endocrine disrupting compounds. It is also necessary for recovering useful products like water, treatment chemicals, energy, and fertilizer.
All sewage is collected through high-specialized wastewater collection systems (either combined or separate sewers), which are then transferred to wastewater treatment plants for purification.
Biological wastewater treatment involves the removal of dissolved and particulate biological oxygen demand (BOD) and the stabilization of organic matter that may be present in wastewater using a variety of microorganisms. These microorganisms are responsible for oxidizing dissolved and particulate carbonaceous matter and converting it to usable mineral compounds, like carbon dioxide and clean water.

There are many stages to the effective treatment of biological wastewater. At DESCCO Water Treatment Construction, we carefully consider the effectiveness and efficiency of each stage during the design of any water treatment plant project. The stages of biological wastewater treatment is as follows:

Preliminary Treatment – The removal of wastewater constituents such as rags, sticks, floatables, grit, and grease that may cause maintenance or operational problems with the treatment operations, processes, and ancillary systems of a wastewater treatment plant construction.

Primary Treatment – The removal of a portion of the suspended solids (solids physically suspended in sewage that can be removed by proper laboratory filtering) and organic matter (the waste from homes or industry of plant or animal origin that can be broken down by bacteria)
from the wastewater.

Advanced Primary Treatment – An enhanced removal of suspended solids and organic matter from the wastewater, typically achieved by chemical addition and/or filtration.

Secondary Treatment – The removal of biodegradable organic matter (in solution or suspension) and suspended solids. Disinfection is also typically included in the definition of conventional secondary treatment.

Secondary With Nutrient Treatment – The removal of biodegradable organics, suspended solids, and nutrients (nitrogen, phosphorous, or both nitrogen and phosphorous).

Tertiary Treatment – The removal of suspended solids (after secondary treatment) usually by granular media filtration or microscreens. Disinfection is also typically a part of tertiary treatment. Nutrient removal is often included in this definition.

Advanced Treatment – The removal of dissolved and suspended materials remaining after normal biological treatment when required for various re-use applications.

BOD stands for biological oxygen demand. It is the amount of oxygen required for aerobic biological microorganisms (bacteria that require free oxygen for growth) to decompose organic material in a given water source at a certain temperature over a given time period. In waste treatment language, BOD is most frequently stated as the percentage of waste removed or remaining after treatment.

COD stands for chemical oxygen demand. It is a measurement of the oxygen required to oxidize soluble and particulate organic matter in water using a strong oxidizing agent. The COD level can be determined more readily than BOD, but this measurement does not indicate how much waste can be decomposed by biological oxidation.

The typical processes that occur at biological wastewater treatment plants are as follows:

  • Suspended-Growth – In these wastewater treatment plant processes, microorganisms are maintained in suspension within the liquid being treated.
  • Attached-Growth – In these wastewater treatment plant processes, microorganisms are attached to some inert medium, such as rocks, slag, or specific ceramic or plastic materials.
  • Combined Processes – These wastewater treatment plant processes combine elements of both suspended-growth and attached growth processes
  • Lagoon Processes – In these wastewater treatment plant processes, wastewater is treated through a combination of physical, biological, and chemical systems. Much of the treatment occurs naturally, but some systems use aeration devices to add oxygen to the wastewater. Aeration makes treatment more efficient, so that less land area is necessary.

SRT stands for solid retention time. It refers to the average time that activated sludge solids (sludge consisting of microorganisms, non-living organic matter, and inorganic materials) are in a wastewater treatment system. SRT is an important operating parameter for wastewater treatment plant contractors and the design of water treatment plant projects, as it is crucial in the activated sludge process.

The activated sludge process is a common method of disposal for pollutants in biological wastewaters. In the process, large quantities of air are bubbled through wastewaters that contain dissolved organic substances in open aeration tanks. Bacteria and other types of microorganisms present in the system need oxygen to live, grow, and multiply in order to consume the dissolved organic “food” or pollutants in the waste. After several hours in a large holding tank, the water is separated from the sludge of bacteria and discharged from the system. Most of the activated sludge is returned to the treatment process, while the remainder is disposed of via one of several acceptable methods.
An MBR system is a biological wastewater treatment process where a selected membrane is integrated with a biological process to act as a suspended growth bioreactor. In an MBR system, no solids can pass through the membrane and hence the SRT is defined only by wasted solids. The SRT can be controlled by periodically discharging some of the solids (sludge) from the process.
As a wastewater treatment technology that is highly recommended by trusted wastewater treatment plant construction contractors, MBR is considered to be a superior technology compared to conventional activated sludge processes. The main advantages of MBR technology compared to conventional activated sludge process are a smaller aeration tank and footprint due to the higher mixed liquor suspended solids concentrations and also higher volumetric loading rates, lower sludge production, and improved effluent quality as a result of membrane filtration.
An Extended Aeration Treatment System is a biological wastewater treatment process that utilizes an altered Activated Sludge Process, and is typically used to process smaller amounts of wastewater as it only requires a single aeration tank. In this process, oxygen is disbursed into wastewater that is held in a single open aeration tank. Aeration encourages the microorganisms present within the treatment system to grow, multiply, consume, and digest the dissolved organic pollutants within the wastewater. Because this is a single tank system, “aged sludge” is present. This system requires a lengthier mixing period than the conventional activated sludge process. The extended mixing time combined with the presence of aged sludge creates a microbiological ecosystem that is stable and better suited to treat fluctuating waste loads for locations where occupancy levels vary.
Sequencing Batch Reactors (SBR), also known as Sequential Batch Reactors, are a type of Activated Sludge process used to treat sewage in batches. In this process, a mixture of activated sludge and wastewater are aerated within the SBRs in order to reduce the amount of organic matter within the mixture.