Treating and reusing greywater

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Re-watch the ANCHOR lunch talk about treating and reusing greywater here, and consult the slides here.

Let’s draw a bigger picture of challenges to today’s water management systems. We discern climate change, water scarcity and the need to rethink wastewater systems to adapt to these water management challenges. Separate collection of greywater is one such rethinking. It represents at least 65 -70% (and up to 90% if vacuum systems are used for collecting blackwater) of the potable water use in a household. The greywater can be divided into weak greywater from showers and sinks and heavy greywater from laundry machines and kitchen sinks. 

Greywater is suitable for water reuse. Such reuse lowers the pressure not only on our drinking water sources, such as groundwater, but also on our wastewater infrastructure. The latter can lead to less sewer overflows and consequently less micropollutants and pathogens in the environment. Greywater treatment could also save costs: simpler treatment, heat recovery, water reuse. Potential reuse applications are versatile: toilet flushing, irrigation, environmental recharge of natural waters, and reuse in industrial processes.

The H+ demo in Helsingborg holds three pipes that pass blackwater, greywater and food waste separately to RecoLab. This is the resource recovery facility located at the wastewater treatment plant in Helsingborg. Its greywater treatment train is quite advanced, containing a collection tank, an anaerobic tank, an aerobic step, a sedimentation step, a drum filter, a nano filter, an ozonation unit and a heat exchanger.

Operational challenges did emerge, for example the growth of filamentous bacteria, which causes poor settling and making it hard to recirculate the sludge. The water quality after treatment is, however, showing good results; the biology is working and is removing some nutrients and COD. But when the sedimentation does not work, the treatment works less well. The nano filters help ensure a good water quality after all treatment steps. The water quality after treatment fulfills most of the EU regulation concerning chemical content for discharge, bathing water quality, irrigation and drinking water. The one parameter that is unclear at this point is for irrigation class A and that is mostly because it has BOD removal demands and BOD is not measured at RecoLab for at this point. The measured COD data suggest that the BOD limit value is met but it has not been confirmed. There have been problems with leaking membranes and causing pathogens in the effluent. The membranes are now mended, and it is expected that future analyses will show that the effluent water meets standards up to drinking water quality.

The planned reuse (to meet drinking water standard) in Helsingborg is to top up the swimming pool under construction in close proximity to RecoLab. There is also a possibility to use the treated greywater to replace the drinking water used within the processes in the conventional wastewater treatment plant.

In summary: (i) greywater can be treated to meet high effluent quality standards regarding chemical parameters, (ii) biological treatment of greywater needs more research and development, (iii) more research is needed for compliance with pathogen regulation, (iv) high potential for reuse but unclear legal landscape. We also need more sites working on greywater reuse to develop knowhow.

The Jenfelder Au greywater treatment train contains an underground storage tank (10 m³), a screw sieve, a small receiving tank (1 m³), a distribution pipe to keep the water at the same level in the treatment, a fixed bed bioreactor (400 m² surface), two ultrafiltration units (each 50 m²) and a permeate tank (0.5 m³). From the pilot plant three types of reuse are tested: pond recharge in the ANCHOR project, and toilet flushing and irrigation in another project.

With the additional ultrafiltration membrane provided through ANCHOR, about 25 m³ of permeate can be produced per day. The produced service water, or permeate, meets the German standards for the reuse from the pilot plant (irrigation, toilet flushing, pond recharge).

De Nieuwe Dokken is a city district in Ghent with 400 housing units. The project collects blackwater, kitchen waste and greywater for onsite treatment. Right now about half of the housing units are built and occupied.

The treated water is reused locally by a soap factory located next to De Nieuwe Dokken, from which the city district receives residual heat that otherwise would go to waste. Truly an urban-industrial symbiosis! Water quality requirements from the soap factory have to be met, e.g. on organic and nutrients content as well as on heavy metals, pathogens and colour. Several of the parameters are similar to those for drinking water, which means it is challenging to meet for an onsite operator. However, they put in the work to achieve the required quality! Colour and pathogens are the parameters to be most careful about, to deliver the quality necessary for the soap factory.

The water that the soap factory cannot reuse, is sent to refill a rainwater pit, and the third option is to discharge to surface water. 

The parameters for reuse by the soap factory guided the development of the processes in the onsite treatment system. There are two distinct streams that are collected and treated separately in the beginning of the treatment train: blackwater (with shredded kitchen waste) and greywater. The concentrated blackwater (with shredded kitchen waste) goes through an up-flow anaerobic digester (UASB) for production of biogas, after which struvite is precipitated. The decantate remaining then joins the greywater treatment train at its beginning. The greywater (and the blackwater decantate) goes through a denitrification-nitrification process where also P and COD are removed. After this process the water goes through an ultrafiltration, an activated carbon step (mainly for colour removal), heat exchanger and finally a chlorination step. The treatment train has a multiple barrier approach to achieve the quality demanded by the soap factory.

The potential for water, heat and struvite recovery once the area is fully developed is 60 m³/day, 600 MWh/year and 1.2 ton/year respectively. Water and heat are most important for the local circular economy.

Within the ANCHOR project DuCoop is working on increasing the autonomy of the onsite treatment facility, minimize the operational costs, prevent sludge wash-out, and guarantee colour-free quality water for the soap factory. In Ghent there is a PLC control with sensors, but it does not measure minute by minute. Within ANCHOR DuCoop are looking into what sensors could be used to guarantee good effluent quality.

Though challenging, DuCoop would still choose to mix the blackwater with the greywater in De Nieuwe Dokken in hindsight.

An important success factor for greywater reuse is to understand the need of water quality for the actual reuse. The higher the quality demands, the higher the costs to produce that water. However, not all reuse alternatives demand drinking water quality. Have aligned agreements on the quality of delivered water and also on the water uptake. Another important success factor is to keep it simple – each process step adds potential for failure and increased maintenance. It is important to choose a treatment train that maximizes benefits for the needs. Avoid colour challenges –blackwater adds more colour than greywater so don’t mix the flows if colour matters. Finally, optimize with automation to ensure robust water quality with minimal monitoring.

It is the effluent quality that determines what the treated greywater can be reused for, not the type of greywater per se. The question is to which level you treat the greywater, the quality of the treated water and the barriers you have to ensure quality. The type of greywater you have, however, will impact what the treatment train will look like to achieve the desired effluent quality.

There is a dialogue between DuCoop and the soap factory for the maximum reuse of water. Two parameters are important in that dialogue: the dynamic availability and when the water is needed in the industrial process. It is possible that there is a need for higher buffer capacity to equal out between these parameters, which will increase the water reuse. This extra buffer capacity will, in that case, be covered by the soap factory.

At the scale of RecoLab, it is much easier from a legislative, economic and building perspective to have one pipe out from the treatment facility to one customer approved than to pipe it back into the neighbourhood. Also, the amount of water that can be reused for toilet flushing in a vacuum system is minimal (1L/flush). In Belgium, on the other hand, there is a culture of rainwater reuse and Flanders now requires a secondary circuit in new houses for rainwater reuse. Therefore, projects in Belgium that discuss greywater, the logic for rainwater reuse is adapted. Hence, secondary circuits for greywater reuse are quite accepted.

If we do mixed wastewater reuse it can be much more challenging to treat and to recover resources. If you treat only greywater locally it can be quite simple to treat to a quality suitable for e.g. irrigation. If the blackwater is collected separately it facilitates recovery of more nutrients in cleaner fractions compared to a mixed wastewater. In an expanding city local treatment of greywater (being the absolute bulk of the domestic wastewater) can allow for city district development in spite of existing pipes and wastewater treatment plants reaching its capacity. Hence, there are a different drivers to go for source-separating wastewater systems, especially for new areas. This does not advocate reuse from mixed wastewater, given that we have so much existing conventional wastewater infrastructure.

In De Nieuwe Dokken there is no specific use at this point. In RecoLab there has been a pellet produced from the struvite and ammonia sulphate, which has been field tested by farmers, with good results. Currently not enough struvite and ammonia sulphate is produced to actually make a commercial pellet, so whatever is produced is gifted to the company making the pellets and they use it in their research. The water utility in Helsingborg is, however, engaged in a process to end-of-waste classify the struvite and ammonia sulphate with the purpose of production of a commercially valuable fertilizer in the future.

This lunch talk highlighted that the different ANCHOR projects have different ways to think about greywater and its reuse. The project demos are therefore complementary, and we are developing a portfolio of both technical and organizational solutions. We are most grateful for our audience, bringing out many diverse aspects beyond greywater treatment and reuse.

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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).