Department Process Engineering

This introductory booklet provides an overview of the Circular Sanitation Toolbox and its parts. 

Urine diversion is the separation of urine from the rest of the remaining wastewater streams. This strategy helps divert nutrient loads from existing treatment plants and can yield nutrient-rich fertilizers.

Dry sanitation refers to the collection of excreta or feces without flush water, contributing to water savings. Treatment of the excreta or feces yields soil amendments.

Vacuum sanitation is the collection of excreta via vacuum piping under negative pressure, with minimal water for flushing. In addition to saving water, liquid and soild fetilizers, as well as soil amendments can be recovered. The organics can be valorized to produce biogas.

Flush sanitation refers to the standard practice of using flush toilets to handle human excreta. Treatment allows for the recovery of water from the liquid fraction, and nutrients and organics from the solid fraction.

Water reuse includes the collection and treatment of greywater, or mixed wastewater, for non-potable or potable reuse of the treated water. Water reuse results in drinking water savings.

Water-efficient fixtures include applicances and fitting that reduce water demand. It fixtures reduce hot water demand, energy demand for heating is also reduced.

Rainwater harvesting refers to the collection of rainwater from above-ground surfaces (e.g.,rooftops) during rainfall events. Rainwater can be used for non-potable and potable applications, replacing drinking water demand.

Heat recovery refers to the use of heat exchangers or heat pumps to recover heat from greywater or mixed wastewater to save energy.

Extended storage of urine is the simplest, cheapest and most common method to hygienize urine. Urine storage achieves partial pathogen inactivation due to the combination of increased pH, temperature and time. Recommended storage time is at least six months. To enhance pathogen inactivation and reduce treatment time, the temperature of stored urine can be raised. This process is called pasteurization.

Nitrification-Distillation of urine (know as the VUNA process) treats urine to produce a concentrated liquid fertilizer. It combines biological treatment to stabilize nutrients, activated carbon filtration to remove micropollutants, and vacuum distillation to concentrate and hygienize the output. This high-tech solution is best for urine volumes of 500L/day or more.

Struvite precipitation is a nutrient recovery technology that uses a chemical reaction to form magnesium ammonium phosphate hexahydrate (MgNH4PO4 •6H2O), or MAP. High recovery of phosphorus (90%) can be achieved in many applications. Struvite is a bioavailable, slow release fertilizer that can be stored, transported and easily applied to fields.

Composting is the microbial degradation of organic matter under aerobic conditions to a humus-like product: compost. On-site composting, often in chambers or containers, achieves mesophilic conditions (20-45 °C), but not thermophilic conditions (45-70 °C). Vermicomposting, relies on both earthworms and microorganisms for the degradation of organic matter. The resulting compost can be used as a soil amendment if properly stabilized and sanitized.

Composting is the microbiologial degradation of organic matter to a humus-like stable product. Off-site composting, where waste is collected and transported to a dedicated facility, is often larger and more controlled compared to on-site setups. Off-site composting often occurs in aerated static piles, windrows (tuned regularly), or in-vessel systems (containers with precise control).

Anaerobic digestion is the degradation of organic matter by microorganisms in the absence of oxygen. The technology is well-suited for wastewater and wastes with high levels of organic matter The technology yields biogas, a mix of 50-75 % methane (CH4) and 25-50 % carbon dioxide (CO2), a reduced and stabilised sludge volume, and an effluent.

Settling tanks and screens are used for the separation of large solids and suspended matter from wastewater streams. These technologies are generally implemented as a primary treatment step to protect and/or to simplify subsequent biological or advanced treatment steps in water treatment and reuse.

Vermifilters treat blackwater or mixed wastewater in a multi-layered system that includes solid-liquid separation, vermicomposting of the solid fraction, and aerobic biological treatment of the liquid fraction. The solids are broken down by earthworms and microbes into compost, while the liquid passes through porous layers for further biological treatment.

Constructed wetlands are engineered systems that mimic natural wetlands to improve water quality. Water flows through or over a substrate planted with reeds and plants, where microbes and plants work together to remove nutrients and contaminants. Wetlands are low-tech, nature-based solutions that in addition to treating water, offer ecosystem services, and have recreational and aesthetic value.

Trickling filters are aerobic biological reactors where wastewater flows over a fixed bed of media (e.g., plastic, gravel, lava rock), allowing biofilms growing on the media to degrade organic matter. They operate passively or with energy input for recirculation, and require good airflow and drainage to maintain aerobic conditions.

Aerated bioreactors make use of microorganisms growing in flocs (i.e., activated sludge), and/or biofilms on fixed beds or moving carriers, to degrade organics and remove nutrients from wastewater. Active bubble aeration maintains aerobic conditions and keeps the activated sludge and/or carriers suspended. Variations include SBR, MBBR, SFBBR, IFAS, and membrane bioreactors (MBR). 

Membrane bioreactors combine biological treatment with membrane filtration to produce high-quality treated water. Microorganisms in the aerated bioreactor degrade organics and nutrients, while micro- or ultrafiltration membranes remove solids and pathogens. MBRs are compact and well-suited for contexts where space is limited.

Membrane filtration is a pressure-driven separation process where water is pumped through a semi-permeable membrane to filter out contaminants. Membrane pores allow water to filter through, but hold back contaminants that do not fit through the pores. Membrane units are often used as a filtration step after biological treatment of greywater or mixed wastewater, or for the treatment of rainwater.

Granular media filtration removes suspended and colloidal particles by passing water through layers of granular materials like sand, activated carbon, or gravel. Particles are retained by surface straining, adsorption, ion exchange, or settling. Filters can be pressure-based (closed system) or open sand filters using gravity. 

Ultraviolet (UV) disinfection and chlorination are treatments to inactivate pathogens, including bacteria, viruses and protozoa present in the water. UV disinfection uses light to damage pathogens’ DNA, preventing reproduction; chlorination adds chlorine to water to kill pathogens chemically. Chlorination is the only treatment that provides residual disinfection, preventing regrowth of pathogens in water distribution networks and storage tanks.

Ozonation and other AOPs can be used for the targeted removal of organic micropollutants (like pesticides, pharmaceuticals and PFAS) from treated water for potable reuse applications, or when stricter discharge regulations apply.

All of the case studies included in the toolbox have been organized into a speadsheet to provide a quick overview of and allow for comparison between case studies. The database also includes project partners (engineers, planners, technology providers and other stakeholders) involved in each case study (where available).