Municipal Wastewater

Influent Monitoring

Preliminary and Primary Treatment

The wastewater treatment process begins by removing large solids and other material during preliminary and primary treatment stages:

Preliminary Treatment

Preliminary treatment is typically the first process within the treatment plant and consists of two categories:

• Screening: This is the process of removing trash, rags and other debris by passing the water through a mechanical screen or rotating drum.
• Gritting: Grit is removed by reducing the velocity of the wastewater, so that heavier inorganic particles like pebbles and sand can settle to the bottom and be removed via gravity.

The grit and screenings are washed and compacted before being collected in a dumpster. These preliminary processes are very important for modern water resource recovery facilities, as grit and debris can irreparably harm downstream processes including primary clarifier mechanisms, aeration diffusers and membranes

Customer Challenges

Influent Management

Influent to Primary Clarifier (Post-Screen)

Secondary Treatment

Secondary treatment removes the soluble organic matter, nutrients such as nitrogen and phosphorus and most of the suspended solids that escape primary treatment.

Biological Secondary Treatment Processes

Most often, biological processes are used in which microbes metabolize organic compounds and nutrients to grow and reproduce. There are two common biological secondary treatment processes:

• Suspended growth systems
• Attached growth systems

Suspended Growth Systems

A suspended growth process fosters the growth of suspended microorganism flocs from individual organisms already present in the wastewater and in the return activated sludge.

The flocs contain organisms that can remove the pollutants through aerobic, anoxic and anaerobic environments. Once the pollutants are removed, the flocs are sent to a secondary clarification process where they separate from the water via gravity. A portion of sludge in the bottom of the secondary clarifier is then directed back upstream to blend with the primary effluent (Return Activated Sludge) to create mixed liquor. The remainder of the sludge is removed from the process (Waste Activated Sludge) to create the ideal ecology of microorganisms.

Attached Growth Systems

Attached growth systems rely on the microorganisms to attach to a media and create a biofilm. The settled sewage is either mixed or sprinkled over the biofilm coated media where the microorganisms remove the pollutants. Like the suspended growth process, biofilm fragments and suspended flocs are sent to a secondary clarifier for separation where sludge is recycled and wasted and clean water is discharged to the next process.

Organisms Require Nutrients in a Balanced Ratio

For biological treatment to function efficiently, organisms require nutrients in a balanced ratio, including:

• Carbon
• Nitrogen
• Phosphorus (referenced as C:N:P)

Additionally, the following elements are typically present in wastewater:

• Iron
• Copper
• Zinc
• Nickel
• Manganese
• Potassium
• Sulfur

The commonly accepted C:N:P Ratio is 100:5:1, although some facilities thrive outside of this ratio, while others experience polysaccharide slime formation or filamentous bacteria growth that inhibit the biology and settling in the secondary clarifier.

Multiple biological processes can be employed to complete secondary treatment. These processes include:

• Plug flow aeration basins
• Complete mix aeration tanks
• Sequencing batch reactors
• Oxidation ditches
• Trickling filters
• Moving bed biological reactors
• Integrated fixed film activated sludge

Biological Nutrient Removal (BNR)

Biological Nutrient Removal (BNR) alters the environment of the microorganisms to remove nitrogen and phosphorus from the water. A BNR process consists of anaerobic (no oxygen or nitrate), anoxic (no oxygen, nitrate is present) and aerobic (oxygen present) stages, during which the water is moved through a series of chambers to perform various biological functions.

Chemical treatment processes can also be used, such as the chemical removal of phosphorus. By introducing a chemical precipitant within the aeration basin and clarifiers, phosphorus is removed by flocculation, binding into insoluble compounds that settle out and can be removed as sludge.

Customer Challenges

Phosphorus Management

Organic Management

Managing Energy Costs

Over aeration impacts downstream processes; Unexpected organic slug loads

Secondary Clarification

Tertiary Treatment

In tertiary treatment, techniques such as filtration, disinfection, carbon absorption and other processes are used to remove the remaining organic load, suspended or dissolved solids, pathogens and heavy metals that pass through other treatment processes.

Also called effluent polishing, tertiary treatment raises the effluent quality to the level suitable to its intended use, whether for discharge into lakes, rivers, oceans, reuse as non- crop irrigation (parks, golf courses, greenways, etc.), groundwater recharge or in certain cases, as influent to drinking water plants.

Wastewater plant effluent must be monitored to ensure compliance with effluent permit limits, which vary based on jurisdiction and country.

Chemical Dosing

Polish the Effluent for Micropollutants

Sludge & Solids Management

The method for handling the sludge removed from the process depends on the volume of solids as well as other site-specific conditions. Aerobic digestion is often used by facilities less than eight million gallons per day of inflow. Waste Activated Sludge and if present, Primary Sludge, are added to an aerated reactor where microorganisms feast on the organics and microorganisms present in the sludge to reduce the volatile solids content and the overall mass of sludge.

Anaerobic digestion is typically used at facilities greater than eight million gallons per day of inflow and involves the use of sealed reactors to create an anaerobic environment for different organisms to feast on the organics and microorganisms in the sludge through the processes of acidogenesis and methanogenesis. The methane formed by anaerobic digestion can be used to fuel boilers to heat the digester, flared, or cleaned and repurposed as a green energy source.

Thickening involves concentrating the sludge by removing a percentage of the liquid portion by adding polymer compounds, and is often employed before anaerobic digestion. Dewatering with belt presses, centrifuges or other means further concentrates sludge into a cake. The cake can be further dried, or simply disposed through land application or landfills.

Customer Challenges

Secondary Treatment Stage

Sludge Blanket Management: Primary/ Secondary Clarifiers and Gravity Thickeners

• Primary Clarifier and Gravity Thickener: High sludge blankets can become too dense to pump. If the blanket is too low watery sludge can cause issues with other processes
• Secondary Clarifier: High blanket levels can lead to increased and unwanted solids and turbidity in effluent. Unwanted secondary clarifier denitrification and phosphorus release can occur from high blankets

Return Activated Sludge and Waste Activated Sludge

• Return Activated Sludge (RAS): High return rates can lead to higher electrical consumption due to excess pumping. High return rates can affect wasting efficiency.
• Waste Activated Sludge (WAS): Understanding the solids concentration of WAS is key to managing a proper Solids Retention Time (SRT)

Mixed Liquor Suspended Solids (MLSS)

Once a day grab sample offers an incomplete picture of the process: Inaccurate visibility to daily concentrations leads to confusion about current process conditions, introduces error and lag time for process control, grab samples may not be representative, lab test may have been improperly performed, and inconsistency between operators and or shifts.

Secondary Treatment Stage

Understanding sludge age, seasonal, and diurnal changes for effective treatment

• Inaccurate/incorrect SRT steals treatment capacity which can lead to compliance issues – BOD, Ammonia, Phosphorus, Total Nitrogen
• Inaccurate/incorrect SRT can lead to filamentous bacteria concerns: Filaments can create bulking and foaming issues. When chlorine is used for controlling overgrowth it also inhibits good bacterial composition.

Thickening & Dewatering

Thickening: Optimize Feed stock of the Digester

• Sludge with excess water can lead to a short hydraulic retention time in the digester which reduces digester stability and biogas production.
• Thickening and Dewatering: Costs of Operations
• Low sludge cake dryness (% solids) can lead to higher sludge disposal and hauling costs.
• Un- optimized polymer dosing and system settings leads to higher operation costs.

Effluent and reuse

While effluent from wastewater treatment facilities is commonly discharged to the environment in rivers, oceans, or other bodies of water, there are a variety of other options for discharge. These include:

• agricultural irrigation
• use in parks and recreational facilities (golf courses and sports field irrigation, snow making)
• wildlife habitat or aquifer/wetland/marsh recharging
• industrial uses such as process water
• street cleaning
• direct potable reuse where water is sent to a drinking water plant for further treatment.

Customer Challenges

Effluent Regulatory Compliance

Effluent Monitoring

Reuse

Disinfection

Water Intelligence

Regulations

Regulations and permit limits vary from region to region and country to country. Generally, target limits are established by the acting authority, and plants measure and report to demonstrate compliance with established limits. In this way, important parameters that indicate water quality and environmental and health standards are monitored and maintained, ensuring the continued health and safety of the populations being served.

For information regarding regulations in the US: https://www.epa.gov/npdes

For information regarding regulations in the EU: https://www.eea.europa.eu/themes/water/european-waters/water-management

Effluent and reuse

Although effuents from sewage treatment facilities ar commonly discharged into the environment, into rivers, oceans or other water bodies, there are several other options for their disposal. These options include, but are not limited to, agricultural irration, use in parks and recreational facilities (irrigation of golf courses and sports fields, snowmaking), wildlife habitat, or recharge of aquifers, wetlands and swapms, industral uses such as process water, street cleaning or the reuse of potable water when it is returned to a plant for futher treatment.