Collecting and treating blackwater for resource recovery

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Re-watch the ANCHOR lunch talk about collecting and treating blackwater for resource recovery here, and consult the slides here.

Blackwater contains both resources and hazardous substances. By collecting blackwater separately you can both recover the resources and provide barriers better compared to a conventional wastewater system. 

Main resources: nutrients and energy

Main hazards: pathogens, antibiotic resistant bacteria and genes, as well as micropollutants such as pharmaceuticals

Conventional wastewater treatment systems do not provide enough barriers against the spread of the hazards in blackwater, illustrated by Professor Londong by a German study which showed that the largest release of antimicrobial resistance in surface waters is the release caused by sewer overflows. Hence, a separate collection of blackwater provides an important barrier against spreading of its hazards and it also makes it easier to recover its resources.

Blackwater collected with vacuum toilets and sewers is a very smooth wastewater flow to work with:  it has a low flowrate equal over time and it has stable and high concentrations of nitrogen and phosphorus. Moreover, it is also very clean in its own way, with for example very low values of heavy metals.

Blackwater’s core process is anaerobic digestion. Earlier research around improved biogas production has been proven to hold true in the Swedish demo Recolab. Source-separating wastewater systems with vacuum collection of blackwater produce 65% more biogas per connected person in compared to the conventional wastewater system.

The typical blackwater treatment train is hence formed around the anaerobic digestion: first a storage tank, then the anaerobic digestion, a solids removal step in the digestate can be necessary, the nutrient recovery step, and then polishing step if there is direct discharge. 

Only 2% of the flow leaves as sludge, so most nutrients pass the digester and are available for nutrient recovery. In smaller systems the blackwater can be reused, after sanitisation, without specific nutrient recovery, which will allow for reuse of all blackwater nutrients. However, in RecoLab there is phosphorus and nitrogen extraction through struvite precipitation and ammonia stripping. Struvite is included in the EU fertilizer regulation and even struvite from wastewater can be end-of-waste classified and used as a P source in organic farming. The struvite from RecoLab is of excellent quality. It has improved with time, indicating some metal leachate from the equipment in the beginning. Ammonia stripping is high in chemical and energy use but still has a positive effect on climate. However, membrane stripping could be a more efficient way forward for N recovery. The digestate’s nutrients could be concentrated through other methods as well, such as distillation or freeze concentration. The recovered nutrients in RecoLab have been tested with farmers in field trials with good yields.

Polishing is needed if you have direct discharge. And even if there is some pharmaceutical removal in the anaerobic digestion, a pharmaceutical removal step is most likely needed as part of the final polishing given the pharmaceuticals in blackwater.

In summary Hamse underlined that blackwater treatment has moved from university research into municipal operation. As a utility operator Hamse says that “blackwater is the wastewater of my dreams”.

What has value in the streams we are separating and why? Is there a market for that value? Important aspects for markets are location and scale. Each location has its own economy, its own market. Within the economy there is scale – from individual household to city district/city/county. It is important to understand why we make certain decisions in different scales to invest in source-separating systems, to create the correct market for adoption and implementation.

The market itself – where can we create a market for source-separated streams? Three important value streams for recovery are: water, energy, and nutrients. Other value aspects are current infrastructure (sewer capacity, drinking water production capacity, climate change vulnerability). Health and environment parameters matter too: micro pollutants, heat island effects, eutrophication. The important value aspects vary within the scale – for an individual household the most important value aspect is probably how much water can I save, and how much energy can I recover? These aspects will define whether source-separation happens on individual level. If water recovery can be connected to urban greenery it can be relevant for (groups of) individuals’ investments. For a city, the current infrastructure will determine investment in source-separating wastewater systems rather than water savings on household level.

What drives the different markets?  Scarcity (water e.g. in southern Europe, sustainable energy) drives markets but also environmental regulations (e.g. the new Urban WW directive). Also city development where the current infrastructure is running full, together with the possibility to connect green real estate development to water reuse. 

Examples from DuCoop – The blackwater, kitchen waste and greywater are treated locally in De Nieuwe Dokken. Up to 25% of the yearly heating demand can be recovered in De Nieuwe Dokken from the heat in the greywater. Solar panels have also been installed. As for financing the wastewater treatment the cooperative DuCoop gets the wastewater levy on the water bill + the extra revenue from selling the treated wastewater to nearby soap factory Christeyns. Similarly there is a levy on the waste bill which DuCoop uses to run the system. Extra revenue is created from production of biogas.

We also can create societal benefits: the treated greywater can be used for urban greenery to reduce heat island effect with increased well-being for inhabitants and increased property value. Reduction of sewage overflows can also be achieved. In De Nieuwe Dokken there is a connection to the main sewer but for the last 4 years it has not been used often, only during maintenance. New city developments: new developments may be hampered by low capacity in existing infrastructure.

How do we go from concept to implementation? We need a good business model to calculate the different scenarios. The business model in DuCoop is built on 

1. Heating/cooling
2. Governance
3. Electricity and charging stations
4. Water and green infrastructure

In the model we assess CAPEX and OPEX, how quickly will the development happen (= how quickly the revenue streams will develop), and how will we finance it. Modelled DuCoop data show that blackwater treatment and recovery demands more households connected (> 1000) than greywater treatment and recovery (> 200) WHEN heat recovery is included in the greywater model. Density also influences the viability of the business case.

How important is the vacuum system for the functionality of the blackwater treatment in RecoLab? 

The short answer is “very important” because the blackwater treatment processes in RecoLab do not work with blackwater containing too much flushwater. Vacuum system-collected blackwater is 50 times more concentrated than normal sewage.

Retrofitting of conventional wastewater systems with blackwater collection with vacuum can be done either by putting one small pipe, which can be used for vacuum transport of blackwater, into an existing pipe, which is a solution existing on the German market. There is also another relining system, where two pipes are introduced at the same time, where the small pipe is suitable for vacuum systems and greywater can be conveyed in the larger kidney-shaped pipe next to the round small one (for vacuum). Hydraulically this can work for 5-storey buildings. 

Is a buffer tank needed before the anaerobic digestion and how much sedimentation do you get in the buffer tank? Both Ghent (BE) and Helsingborg (SE) have buffer tanks. It adds some security if the system is malfunctioning. It is not costly, the size in Helsingborg is 1/5 of the size of the digester. The content is completely mixed in Ghent and Helsingborg. Hence, no sedimentation! Moreover, traps and/or shredding pumps are necessary in the blackwater system to deal with e.g. wet wipes.

Do you need a UASB for blackwater?  Yes, you do need a technology that separates out the solids from the liquids so that the solids retention time is longer than the hydraulic retention time. There have been some problems with solids washouts in Helsingborg but the added drum filter works well. The drum filter is very low in maintenance – every 6 months there is a cleaning with a weak acid.

With the new EU regulation for fertilizers one can go through end-of-waste classification for many recovered products, including struvite and ammonia sulphate. When a product has reached end-of-waste classification it is instead governed by regulation for chemical products. Helsingborg is currently going through that process for its recovered nutrients. It is, however, also OK to spread waste on farmland – sewage sludge is a waste that can be used on farmland, as long as requirements are met. 

ANCHOR is gathering actors working with blackwater collection with vacuum systems and separate greywater treatment, so in this project there is no one working with urine diversion, nor with dry collection of faeces. However, several urban urine diversion projects exist, e.g. in France and Switzerland, and also EU projects generating more knowledge about urine diversion, e.g P2Green. When it comes to the views in the ANCHOR group on dry faeces collection, Hamse underlined that for the Helsingborg case it was desired that the experience on the household level should not be much different from in a conventional apartment, hence dry collection of faeces would not have worked being too different. With a vacuum toilet the user experience is not much different and very little water is used (1L/flush). Since the food grinder system doesn’t use much water either (4-5 L/p/d), the water use per capita per day for the blackwater and food waste collection is really low.

ANCHOR has a work package focused on investigating acceptance for the systems in the participating demo sites. German experiences have shown that with poor installation the toilets are noisy and user acceptance has been low (the earliest projects about 20+ years ago), but with improved installations there is also increased acceptance. 

What we take with us from the interaction with the audience during this lunch talk is that there is a lot of interest in technical aspects of blackwater systems, but also for legal issues and user acceptance. The work packages in ANCHOR will shed more light on technical aspects and user acceptance, so stay tuned!

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For any questions about this article you can reach out to the author, Elisabeth Kvarnström (Ecoloop, working with Stockholm Stad, SE).