Nature-Based Solutions for Water Management in the Peri-Urban

Photo Stories

Nature-based Solutions for decentralized greywater treatment in Stockholm Archipelago, Sweden: The case of Norrtälje Municipality

Author: Nandita Singh

Sweden is known for its many archipelagos along the coast. Stockholm Archipelago is the largest of these and the second-largest in the Baltic Sea Region. It is said to comprise more than 24000 islands, islets and sherries, covering an area of about 1700 Norrtälje is a coastal municipality, situated in the northern part of the Stockholm Archipelago, comprising about 10,000 of its islands, islets and sherries. Once the refuge of farmers and fishermen, today Norrtälje is one of the most popular summer destinations in the region that attracts Stockholmers and tourists alike. The Municipality has the largest number of summer cottages (or holiday homes) – as many as 13900. While the natural beauty and leisure opportunities available in this summer destination are unparalleled, water shortage as well as degraded water quality are emerging as important sustainability concerns in recent years.

An important reason behind the problem is that many of the summer cottages lie outside the reach of municipal water supply and sewerage network, leaving the responsibility to the property owners. This in turn poses challenges of access to safe water in adequate quantities, and also that of safe disposal of the wastewater generated in these cottages. Wastewater comprises ‘greywater’ which is the used water flowing from sources such as showers, washing machines, and bathroom sinks, and ‘blackwater’ which includes the used water mixed with fecal matter from the toilets. Unsafe disposal of both these kinds of wastewater due to inadequate or inappropriate treatment poses risk to the safety of the local surface- and groundwater resources, besides contributing to eutrophication (excessive increase in nutrients and minerals) of the Baltic Sea. The conversion of a large number of the summer houses into permanent residences and impact of climate change on the precipitation pattern further aggravate these problems. The situation in Norrtälje is only a mirror of the situation in the whole country where a total of about 700000 properties lack connection to municipal sewerage. Approximately 70% of these are used for permanent residence, while the remaining are leisure properties.

In Sweden, there is no special law that regulates decentralized wastewater facilities. Instead, there exist only general norms to consider which imply that whoever is responsible for an activity with emissions is also responsible for minimizing the damage caused by the emission or reducing the risk of so-called inconvenience to environment and human health. In this light, for promoting sustainable living as well as tourism in Norrtälje, as also in other highly acclaimed summer holiday destinations, there is need to promote practices that can protect the local water resources and the Baltic Sea and enhance water sustainability in general. Since ‘greywater’ often represents over 60-70% of the household wastewater but requires simpler treatment (compared to blackwater) before being released into the environment, a focus on greywater in this context is important. There is need to educate and sensitize the local residents / owners of the summer houses about how they can contribute to addressing the problem by adopting more sustainable greywater disposal solutions. Through the case of Norrtälje Municipality, this photo story aims to make a presentation of the possibilities offered by ‘nature-based solutions’ (NBS) for greywater treatment in the Stockholm Archipelago. The title photo depicts a summer cottage near water amidst beautiful, serene settings in Norrtälje municipality, located in the Stockholm Archipelago (Photo courtesy: Visit Roslagen).

A summer cottage in Norrtälje amidst beautiful, serene settings near water (Photo courtesy: Norrtelje Tidning)

Norrtälje municipality is responsible for environmental and water planning within its territory. The planning is governed by a number of policy frameworks, particularly the EU Water Framework Directive, the Baltic Sea Region Action Plan, and the Swedish Environmental Code, 2000. However, most of the summer cottage areas lie outside of municipal planning areas (and thus can be classified as ‘peri-urban’). Hence, the individual cottage owners are obliged to organize onsite wastewater treatment on their own. These cottage owners, maybe either permanently residing in their properties or visit during holidays. They could even be renting out their houses to tourists during the year. The wastewater treatment may be organized individually at a household scale or collectively through neighborhood associations, but always through self-financing. In either case, the cottage owners are responsible for ensuring that the onsite solutions adopted meet the standards specified within the scope of the relevant policy frameworks. These small-scale systems are granted approval by the municipality that also helps with information dissemination through digital and other platforms. A number of different kinds of solutions are in use, many of these can be classified as ‘nature-based’, being solutions that are inspired by processes and functioning of nature. Some of them simultaneously provide environmental, social and economic benefits as well.

‘Infiltration’ as a NBS for greywater treatment (Photo courtesy: Avloppsguiden)

The first kind of NBS for greywater treatment is "ground-based purification” where soil is used to clean the wastewater. There are three different variants of this technology, namely, infiltration, soil bed, and biomodule. A common requirement for ground-based greywater purification is the sludge separator for mechanical purification, which is often a two-chamber well. The figure presented above denotes the option of “infiltration” where the greywater is first led to a sludge separator, from where it is led or pumped to the infiltration bed. When the dirty water infiltrates, a biofilm of bacteria forms in the soil just below the sprinkler lines. The bacteria break down pollutants and infectious agents (biological purification). The purification of the phosphorus takes place via the fixation and chemical binding to soil particles in the bed (chemical purification). The purified wastewater then goes further down to the groundwater. Infiltration is not suitable for all kinds of plots, as the purification requires the right kind of soil conditions. This NBS involves relatively low investment cost and is safe to operate. However, it poses risk of groundwater contamination, as it is difficult to verify that the plant cleans as intended. Also, it requires sludge emptying at least once a year.

‘Soil-bed’ as a NBS for greywater treatment (Photo courtesy: Avloppsguiden)

If the soil on the property is not suitable for infiltration or if infiltration of greywater is not desirable due to multiple drinking water sources nearby, ‘soil bed’ is an alternative NBS to use. A soil bed functions like an infiltration facility, but the purification takes place in a built-up sand layer instead of in the ground's natural soil layer. There are soil beds with a dense bottom and soil beds with a loose, open bottom. The dense bottom, for example a rubber sheet, means that any water that flows through the soil bed cannot infiltrate but collects and is led out, for example, into a ditch. Thus, dense bottom poses lesser risk of groundwater contamination. In soil beds without a dense bottom, a part, or a large part, of the wastewater always infiltrates. Like infiltration, soil beds involve relatively low investment cost and are safe to operate. Sludge emptying is required at least once a year and the facility needs a space of about 20-50 sq.m. As a rule, additional phosphorus purification may be needed after the soil-bed. For this, ‘phosphorous filter’ is a good nature-based option where phosphorus in the wastewater gets bound to a material with a high capacity for phosphorus binding, lime being a common option. Spent filter material contains phosphorus and can be used as liming and fertilizer on arable land or for soil preparation.

‘Biomodule’ as a NBS for greywater treatment (Photo courtesy: Avloppsguiden)

Biomodule comprises a tubular support material installed in the soil where biological purification of greywater takes place

Biomodule is a solution where biological purification of the greywater takes place according to the same principle as in a soil bed or infiltration, but within a tubular support material which is installed in soil. In these tubes, a biofilm is formed consisting of bacteria and fungi that break down infectious agents and organic material and oxidize ammonium to nitrate. Biomodules can be used as a compact soil bed with a layer of sand on which the modules are placed and a sealing layer at the bottom. Discharge then occurs to surface water. An advantage is that it can be built on a smaller surface than a conventional ground bed, approx. 10-15 sq.m. The plant can also be built as an enhanced infiltration, with an infiltration surface under the filter. Properly installed biomodules are generally robust installations that are relatively insensitive to disturbances, and they require very little supervision and maintenance. There may be need to install additional phosphorous filter after the biomodule, to remove phosphorous since phosphorous removal in the biomodule is small.

Greywater dam – a constructed wetland – where greywater purification works through green phytotechnology

Greywater dam is a NBS based on green phytotechnology and designed in accordance with constructed wetland principles. Here the purification function takes place in a cooperation with microorganisms and bacteria which breaks down the organic waste and plants that take up nutrients and other substances. The purification takes place in a number of steps, where the first is a sludge reed bed where the sludge from the drain is composted, followed by a pond with plants where the water is recycled via a water ladder for oxygenation. In the above photo, the first step is seen in the background while the second step is visible in the front. After these steps, the purified water is spread over a small gravel bed / stone coffin via a wide drain below the ground.

Biological treatment plant as a NBS – with a pre-treatment tank (on the left) containing microorganisms, and
a main chamber (on the right) containing microorganisms and natural aquatic plants (Photo courtesy: Tranes)

A biotreatment plant in operation using microorganisms and green phytotechnology inside a compact tank

Biotreatment plant is a NBS using only microorganisms and green phyto-technology for purifying wastewater. Here, greywater purification takes place in two steps. In the first step, microorganisms inside a pre-treatment tank form a biofilm to degrade the sludge, which minimizes the need for sludge removal. From this tank, the greywater is pumped to the main chamber where it is exposed to other types of microorganisms and to natural aquatic plants whose root systems also make up a good environment for the microorganisms. The plants as well as the microorganisms feed on nutrients in the greywater. In some cases, an additional treatment step is required to achieve extra reduction of phosphorus to prevent eutrophication. Here phosphorus is bound to a lime material. When the filter material is saturated with phosphorus it can be spread on farmland as a fertilizer. This treatment system is very robust and offers multiple benefits, such as low maintenance, direct sludge degradation, freedom from use of chemicals, and even reduction of pharmaceutical residues.

BioKube - a mini treatment plant being installed

In addition to the above options, there exist a number of different kinds of compact greywater treatment filters, known by different names such as the one above. Here, the sludge-separator and a kind of biomodule are combined inside a common tank, and the purification is primarily based on biological treatment through micro-organisms. However, removal of phosphorous, which is the third step of purification, is almost always a chemical process, and hence these solutions are not entirely nature-based solutions.

This photo story demonstrates that for closing the water cycle gap in peri-urban Norrtälje, there exist a number of NBS that can be installed for greywater treatment in the summer cottages. Some of them, such as infiltration, and even soil-bed, have been in place since decades, when the summer cottages first came into being. However, their efficiency has come to be questioned in recent years, since these systems have become old, resulting in increased emissions. Also, of the newer systems, the only NBS which is better known is the biomodule, while greywater dam and biotreatment plant are still quite uncommon. In terms of advocacy for NBS for greywater treatment, website of the company that engages in educating the summer cottage owners about the various options available for onsite wastewater treatment does not discuss about the benefits of NBS for environment, nor does it promote greywater dam and biotreatment plant as NBS. Further, even the Norrtälje municipality lacks any mechanism to spread awareness about the newer NBS technologies for greywater treatment that are being scientifically attested as efficient in function and beneficial for environment, besides being easy to operate and maintain. There continues to be an emphasis on the various ground-based purification systems, all of which are not safe from groundwater contamination perspective.

An analysis of the various NBS options available for use in Norrtälje, or already installed in the summer cottages in the municipality, together with relevant socio-economic dimensions shows that: first, NBS options help jointly address the two major water challenges in the area – quality and quantity, without producing any harmful chemical residues from the treatment process. This contributes to their environmental sustainability. Second, the NBS for greywater treatment bring social sustainability due to the social and cultural benefits these can offer at the local scale. These include not only fulfillment of safe water needs by preventing surface- and groundwater contamination, but also the potential of enriching the leisure environment. Further, adoption of NBS can enhance participatory opportunities to the local residents towards sustainable environmental stewardship. Finally, the NBS also promote economic sustainability through cost savings in the short and long-term, besides potentially enhancing property values.

In comparison to the conventional (non-NBS) technologies for greywater treatment, the NBS options are generally easier to install and their operation and maintenance at household level is simple and low-cost. This enables a good marketing opportunity for NBS and hence, a pro-NBS marketing strategy can help the consumers make sustainable choices. There is need to enhance awareness about the need and long-term benefits of adopting NBS solutions, and also to democratize public participation in environmental stewardship through NBS adoption. There is need to bring policy emphasis on NBS so that specific actions can be initiated to address these barriers.