Through the COVID-19 pandemic of 2020, you may have come across an interesting news story from the Netherlands related to a possible mass testing methodology. Researchers found that sampling and analyzing the raw sewage at the inlet of wastewater treatment facilities could provide clues as to the number of people infected within the catchment area of the wastewater plant. While it is a surprising methodology, it is a sign of times what with wastewater plants which were traditionally considered a resource sink evolving into resource neutral or even resource positive facilities.
Indeed the Water Environment Federation (WEF), the umbrella body for professionals in the water and wastewater treatment field in USA has dropped the term “Wastewater Treatment Facility” from its literature and lexicon and adopted the term “Water Resource Recovery Facility” to highlight and promote this drive. In this piece I take a look at some of the reasons for this shift by providing an insight into the process design basis for wastewater plants.
Why is Wastewater Treatment Important?
Wastewater treatment plants were originally designed to remove organic matter of human fecal origin before being discharged into public water bodies. Organics in the untreated sewage has the potentially to degrade biologically and in the process consume oxygen from the receiving bodies resulting in adverse living conditions for aquatic matter. The wastewater also carries enteric pathogens that if untreated can result in disease outbreak for communities beside the discharge locations. Hence wastewater treatment was and is important from both an environmental and public health perspective.
The nutrients in question are Nitrogen (in the form of Ammonia and organic nitrogen from urine) and Phosphorus (dissolved or particulate). Just as Nitrogen and Phosphorous form important nutrients for human consumption, they also promote algal growth in water bodies that receive nutrient rich wastewater.
These nutrient loads result in a process called eutrophication or the exponential growth of algae that appear as a blue green film on the surface of water bodies and consume available oxygen while blocking sunlight from penetrating the surface. These blooms are unsightly, can stretch for miles and severely impact the aquatic life in water bodies in which they proliferate. In order to remove these organics and nutrients, wastewater engineers employ various microbiological species that consume them for their life and growth processes.
The Solution: Activated Sludge Process
The industry workhorse for the past many years has been the Activated Sludge Process whereby microbial growth of varied species is promoted by feeding on the dissolved organics in one set of tanks. The mass of organisms also has the property of forming flocs in the process tend to incorporate the inorganic non-biodegradable particulates that settle well and. These flocs are led to large clarifiers where the clarified effluent is collected and sent downstream for further filtering, disinfection and discharge. The sludge collected at the bottom has to be further treated before being disposed in landfills.
The microbes that are “employed” in the treatment process require specific environmental conditions to grow that can often times be in contrast with each other. For example, organics are removed by heterotrophic bacteria that reproduce and grow at a much faster rate than autotrophic bacteria which are required to degrade Ammonia (both of which are in the order of days). These competing conditions and the need to promote growth of various types of bacteria as well as have the biomass settle result in wastewater plants having large civil tanks to meet the long hydraulic and solid retention times. At the same time, with wastewater inflows to a plant following a diurnal peaking pattern (up to two times of the average flow for large plants), the tankage required to handle these flows are further increased.
Costs associated with municipal wastewater treatment aren’t just capital intensive. In order to degrade the organics and ammonia, a proportional amount of oxygen needs to be continuously supplied to the wastewater. The amount of oxygen required and the efficiency of aeration system are two important parameters that determine the operating costs associated with the plant. Roughly though, aeration contributes up to 50 – 60% of the operating cost of the plant with pumping forming another 20% of the costs.
Given that wastewater plants can consume anywhere between 20kWh – 45 kWh / per capita per annum every percentage improvement in efficiency is important for a municipality catering to large populations in major towns and cities. While electricity costs are high in much of the developed world, in developing countries these problems can become more acute due to unreliable and discontinuous power supply. Thus with municipalities having to spend public money from taxes to build the plants and then charging the inhabitants a fee to operate them, it is of utmost importance that the requirements and costs to treat wastewater are optimised if not minimised.
Importance of Resource Recovery
With population in cities around the world burgeoning due to economic migration from rural to urban areas, the provision of wastewater treatment systems forms the difference between well maintained public sanitation system or its lack thereof. With a global average of 200Litres and a 60g Organic Load on a per capita per day basis received at the wastewater plants, they will continue to form a big resource sink for municipalities and governments. In this context, the earlier stated shift in perspective from merely treatment to meet public health and hygiene goals to recovering any and every valuable resource from the wastewater to make it self-sustaining while at the same time reducing capital investments is not a luxury but a necessity.
Some of the options that are being studied or have been implemented to achieve the goal of resource recovery are – treating wastewater up to potable reuse standards, extracting energy from the organics to power the wastewater plants, reducing the energy costs associated with nitrogen removal and extracting phosphorous to sell and generate a revenue stream. Future articles will delve deeper into the process design aspects of these systems and look at the maturity of the technologies that can improve resource efficiency.