Similar to the separation of waste in households, wastewater can also be separated at the source, separating the nutrient-rich but small flow of blackwater (wastewater from the toilet) from the voluminous greywater (all other wastewater streams from the household, e.g. from sinks, showers and kitchen appliances). The separate collection and management of blackwater and greywater has major benefits for society particularly regarding the protection of receiving water bodies, the recycling of plant nutrients from wastewater to food production as well as an efficient recovery of energy from wastewater streams.
In such a system, blackwater and greywater are transported in separate pipe systems, requiring an additional pipe (“two pipes out”). Using vacuum toilets and vacuum pipe systems for the collection of blackwater is highly beneficial as they require only a small flush volume (<1L), keeping dilution to a minimum. The highly concentrated stream of blackwater can thus be efficiently transported to a treatment plant for the production of biogas and fertilisers. The greywater is transported in conventional pipe systems and treated to be reused or discharged.
Our current wastewater system
Our existing wastewater infrastructure and management systems are built for linear, and not circular, management, hampering the possibilities of reusing resources connected to our wastewater (nutrients, water, energy, heat, organic matter) in a safe and efficient way, due to dilution and mixing of wastewater flows of vastly different characters. In many cases, stormwater and wastewater are transported in the same pipe; the transport and treatment of the diluted wastewater consumes energy and leads to sewer overflows during storm events. This discharge of untreated wastewater to the environment potentially leads to decreased biosecurity, especially where the natural waters are used as drinking water sources. Today’s wastewater treatment plants are inefficient in recovering heat and energy from the wastewater as most of the heat is lost during transport and most of the organic material needed for biogas production is removed in the treatment processes. The aerobic processes at our wastewater treatment plants need aeration, which is energy demanding and emits laughing gas, a potent greenhouse gas. The recovery of plant nutrients is difficult and is today insufficient. Phosphorus can potentially be recycled to agriculture with sewage sludge but the sludge contains contaminants. Nitrogen can hardly be recovered despite the large demand of nitrogen fertiliser in agriculture. This demand is now met by fertilisers produced using the industrial Haber-Bosch process, which stands for 1% of the global CO2 emissions. The emissions to water caused by today’s wastewater management systems are considerable, not only through overflows but also through the discharge of treated wastewater which contributes a considerable amount of nutrients, organic micropollutants and pathogens to the environment.
Existing implementations of source-separating systems
Blackwater systems have successfully been implemented in several city districts in Europe. Examples include the H+ development in Helsingborg (Sweden), Nieuwe Dokken in Gent (Belgium) and Jenfelder Au in Hamburg (Germany).
Main benefits of blackwater separation in summary
The main benefits of the separate management of blackwater and greywater can be summarised as follows:
1. Enabling the recirculation of plant nutrients
As ca 90% of the nitrogen and phosphorus and ca 85% of the potassium of domestic wastewater is contained in the blackwater, it is an excellent stream to focus on for recovery and recirculation of these resources. While phosphorus recovery is theoretically possible from today’s wastewater treatment plants, the share of nitrogen and potassium that can be recovered is low. In source-separated blackwater the recovery rates of nutrients are considerably higher, potentially increasing e.g. the recovery of nitrogen by more than 400%.
2. Receiving water bodies
As blackwater systems transport the toilet waste in vacuum sewers, the risk for overflows of untreated wastewater is small, decreasing the risk of contamination of potable water sources and the environment with nutrients, pathogens and emerging pollutants.
3. Energy recovery
As the collected blackwater is highly concentrated it can be directly treated with anaerobic digestion, producing up to 70% more biogas than in today’s wastewater treatment plants. Most of the energy in wastewater, however, is heat which is mostly found in greywater (hot wastewater from e.g. showers, dish washers and washing machines) and can be more effectively reused when not mixed with the colder wastewater from the toilets.
4. Water savings
Vacuum toilets used in blackwater systems use up to ten times less flush water than a conventional toilet, saving on drinking water resources.
5. More efficient treatment processes
Most pharmaceutical residues in wastewater end up in the blackwater. With upcoming demands on pharmaceutical removal in the EU, collecting the blackwater separately allows for targeted pharmaceutical removal in a less diluted flow compared with a conventional wastewater.
Blackwater systems thus have the potential to increase nutrient recirculation and food security as well as saving on energy and water and protect receiving water bodies. A cost-benefit analysis of a new city district in Stockholm has shown that source-separating wastewater systems have a higher benefits:cost ratio than the state-of-the-art conventional wastewater system, in spite of the economies of scale working in favour of the conventional system. Because of all the advantages of source-separating wastewater systems it would be desirable that the consideration of these systems is mainstreamed into urban planning and development. That way we can get more systems implemented and build a new more sustainable infrastructure paradigm within the wastewater sector.
Inga Herrmann
Associate Professor in Urban Water Engineering, Luleå University of Technology
Sweden
Elisabeth Kvarnström
Adjunct Professor in Urban Water Engineering, Luleå University of Technology
Sweden
