Modular Waste Water Treatment Solutions
Emerging Trends in Wastewater Treatment
The overarching trends driving global demand for improved wastewater treatment systems are population growth, increasing water scarcity, aging infrastructure along with the associated funding gap and the enactment of stricter water quality regulations to address rising concerns over the effects of inadequately treated water on human health and the environment.
With a large portion of existing wastewater treatment systems reaching the end of their service lives, many countries are faced with the urgent need to fund the replacement of this aging infrastructure. The situation is further exacerbated as new more stringent water quality regulations render many middle-aged treatment facilities incapable of compliance without major upgrades and overhauls.
Because of this, we are seeing the emergence of smaller more economical decentralized wastewater treatment systems capable of meeting current and future water quality standards. Often these decentralized systems are being funded in part by Public Private Partnership (3P or PPP) finance schemes in an effort to bridge the infrastructure funding gap. From a technology stand point these modular packaged systems are generally scalable to accommodate future growth and rely heavily on advanced biological treatment technologies as opposed to chemical treatment options.
Waste Water Treatment Overview
The principal objective of wastewater treatment is generally to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. The three basic types of wastewater are classified by source:
- Industrial Wastewater
- Domestic Wastewater
- Storm Wastewater
The composition of industrial wastewater varies with the type of industrial process discharging the wastewater. Some types of industrial wastewater can be readily treated in conventional wastewater treatment plants. Other more contaminated sources may have to be pre-treated to remove or limit any process-specific contaminants to acceptable levels before being accepted at a municipal treatment facility.
Domestic wastewater comes from normal day to day activities occurring in homes, business, and institutions. It is composed of a variety of organic and inorganic substances. Organic substances consist of molecules that are based on carbon and include fecal matter, detergents, soaps, fats, greases, and food particles. Domestic wastewater is readily treatable in publicly owned treatment facilities, which have been specifically designed to treat domestic wastewater in accordance with prescribed water quality regulations and standards.
Storm water is also readily treated in these municipal facilities as it is relatively low in contaminants. Too much storm water, however, can overload these facilities and the sewage so that biological treatment processes are compromised. Most modern municipal wastewater system designs channel domestic and storm sewage in separate distribution lines.
Measured and Regulated Constituents of Wastewater
Biochemical Oxygen Demand
One of the most widely measured characteristics of wastewater is Biochemical Oxygen Demand or BOD. It is defined as the amount of oxygen aerobic microorganisms must consume to breakdown the organic material present in the wastewater. BOD is determined by measuring the quantity of oxygen consumed by microorganisms during a five-day period. For example, a regulated effluent discharge limit of say 20 ml BOD/l, means that the concentration of oxygen in a water sample diminishes by no more than 20 mg/l over 5 days. Also referred to as BOD5, it is the most common measure of the amount of biodegradable organic material content of wastewater. The BOD of effluent is tightly regulated, because effluent high in BOD can deplete oxygen in receiving lakes and rivers, causing fish kills and ecosystem imbalances.
Total Suspended Solids
Most waste water contains large quantities of undissolved organic and inorganic materials. Referred to as Total Suspended Solids or TSS, these solids are problematic because most are in the form of fine particles, which not only support BOD but also plug up or clog septic systems and mechanical wastewater treatment equipment. TSS are removed from waste streams by various means such as screening, granular filtration, and induced settling/flotation schemes. TSS values are expressed as TSS/liter.
One of the major concerns regarding constituents in wastewater effluent is the concentration of nutrient compounds, particularly nitrogen and phosphorus. When released into receiving waters these nutrients concentrate and stimulate the excessive growth of algae and other aquatic plants, leading to decreased oxygen levels that can lead to hypoxic conditions. Hypoxia occurs when dissolved oxygen concentrations fall below the level necessary to sustain most animal life, generally considered to occur at levels below 2mg/l.
Nitrogen is present in many forms in the waste water stream. Most nitrogen excreted by humans is in the form of organic nitrogen (dead cell material, proteins, amino acids) and urea. Ammonia (NH3) is the primary form of nitrogen in influent entering treatment facilities. Nitrogen removal is accomplished by the biological processes of nitrification and denitrification, which convert ammonia (NH3) into gaseous nitrogen (N2), an inert gas suitable for release into the atmosphere.
Nitrification is a two-step aerobic process facilitated by two different species of nitrifying bacteria. The oxidation of ammonia (NH3) to nitrite (NO2−) occurs first, followed by the oxidation of nitrite (NO2−) to nitrate (NO3−). Then the NO3− is reduced to N2 through anaerobic denitrification. A wide array of bacterial populations is employed in the denitrification process.
The average phosphorus content of most sewage is estimated at around 10 mg/liter. Synthetic detergents are the largest source of phosphorus, followed by human waste in the form of urine and feces. While legislated requirements for more environmentally friendly detergent formulations have reduced the amount of phosphorus entering the waste stream, the contribution from human waste remains constant. Phosphorus removal can be achieved by chemical precipitation or biologically through the enhanced biological phosphorus removal process (EBPR).
In chemical precipitation, soluble phosphorus is transformed to a solid that settles and can be removed with the sludge. Several different metal salt additives are commonly used to chemically precipitate phosphorus. They include Ferric Chloride (FeCl3), Ferrous Chloride (FeCl4), Ferrous Sulfate (FeSO4) and Aluminum Sulfate (alum) (Al4(SO4)3).
Enhanced Biological Phosphorus Removal (EBPR) is a process that uses alternating anaerobic and aerobic zones to provide an environment that encourages the growth of Phosphorus Accumulating Organisms (PAO). PAOs store excess polyphosphate in their cell mass and captured phosphorus is removed with the waste sludge. Essentially EBPR relies on the selection and proliferation of microbial populations capable of sequestering phosphates in greater amounts than would normally be required to sustain their growth. The resulting sludge high in phosphate can be used as fertilizer or disposed of in a conventional land fill.
Basic Wastewater Treatment Processes
The most basic form of wastewater treatment involves the removal of contaminants by allowing or inducing them to either settle to the bottom or to float to the top of reservoirs or clarification tanks for collection and removal. Pollutants are then subjected to biological treatment, either in natural settling ponds or in bio-reactors specifically designed to optimize certain microbial populations and metabolic processes to remove or chemically transform the targeted contaminants.
Wastewater flowing into modern treatment facilities generally passes through five common stages:
- Influent Collection & Delivery
- Primary Treatment
- Secondary Treatment
- Tertiary Treatment
- Disinfection & Effluent Discharge
Influent Collection & Delivery
Influent, as the name implies, is wastewater flowing into a wastewater treatment facility. It contains all the water, debris, and waste that entered the collection system feeding the facility.
Primary treatment involves basic processes to remove suspended solid waste from influent and to reduce biochemical oxygen demand (BOD) – the amount of oxygen microorganisms must consume to breakdown the organic material present in the wastewater. First, influent is passed through a series of raked bar screens to mechanically remove large objects such as bottles, plastic materials, pieces of wood, trash, and other forms of suspended solid waste. The water is then passed through a grit removal system to take out smaller inorganic particles like sand and gravel. Finally, the water flows into large primary clarification tanks or clarifiers, where suspended organic solids either settle by gravity or float to the surface as scum and grease. The floating scum is skimmed off, the settled solids, known as primary sludge, is removed, and the primary effluent moves onto the next stage for secondary treatment. Primary treatment can reduce BOD by 20% to 30% and suspended solids by up to 60%.
Secondary treatment employs aerobic biological treatment processes to remove biodegradable organic matter from the primary influent. Aerobic biological treatment is performed in the presence of oxygen by microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end-products (principally CO2, NH3, and H2O). Several aerobic biological processes are used for secondary treatment, differing primarily in the way oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter.
Secondary treatment systems are classified as fixed-film or suspended-growth systems. Fixed-film or attached growth systems include trickling filters, bio-towers, and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface. Fixed-film systems have been further perfected with the advent of Moving Bed Biofilm Reactors. MBBR systems are the treatment method of choice at Sapphire Water.
Suspended-growth systems include conventional activated sludge processes (also aerated lagoons and aerobic digestion), where the waste flows around and through the free-floating microorganisms, gathering into biological flocs that settle out of the wastewater. The settled flocs retain the microorganisms, meaning they can be recycled for further treatment.
Typically, secondary treatment systems can remove up to 85% of BOD and total suspended solids.
The purpose of tertiary treatment, also referred to as effluent polishing, is to provide a final treatment stage to further improve the effluent quality before it is discharged into the receiving environment. Specifically, tertiary treatment targets remaining bioavailable organics (contributors to BOD), nutrients (nitrogen and phosphorus), and toxins (pesticides, solvents, petroleum, metals – lead, cadmium, mercury). Treatment methods vary and include granular filtration, biological processes such as nitrification and denitrification, enhanced biological phosphorus removal (EBPR), chemical precipitation, coagulation, flocculation, and toxin specific treatment strategies. All in all, tertiary treatment can remove up to 99% of all impurities from sewage, but it is a very expensive process.
Disinfection & Effluent Discharge
The purpose of disinfection is to destroy or inactivate pathogenic bacteria, viruses, and parasites remaining in treated wastewater before it is discharged back into the environment in order to prevent the spread of waterborne diseases.
Various monitoring systems have been established that use the presence of indicator bacteria to gauge the effectiveness of disinfection systems. The most commonly used indicators are total and fecal coliforms, E. coli, and fecal streptococci. The presence of the indicator organisms in numbers greater than a specific target level suggests an increased probability of the presence of pathogenic organisms. The Water Environment Federation reported in 1996 that high levels of coliform indicators have been detected in 88% of the waterborne disease outbreaks in North America.
Chlorine is the most widely used disinfectant for wastewater, followed by ozone and ultraviolet radiation. Chlorine kills microorganisms by destroying cellular material. This chemical can be applied to wastewater as a gas, a liquid, or in a solid form that is similar to swimming pool disinfection chemicals. However, any free (uncombined) chlorine remaining in the water, even at low concentrations, is highly toxic to aquatic life. Therefore, removal of even trace amounts of free chlorine is often needed to protect fish and aquatic life.
Ozone is produced from oxygen exposed to a high voltage current. Ozone is very effective at destroying viruses and bacteria and changes back to oxygen rapidly without leaving harmful by products. Ozone is a relatively expensive approach to disinfection due to high energy costs.
Ultra violet (UV) disinfection occurs when electromagnetic energy in the form of light in the UV spectrum penetrates the cell wall of exposed microorganisms. The UV radiation degrades the ability of the microorganisms to survive by damaging their genetic material. UV disinfection is a physical treatment process that leaves no chemical traces.