NL2029300B1 - Method for the decentralized production of product water from waste water originating from sewers - Google Patents

Abstract

The invention relates to a method for purifying waste water originating from sewage (sewage) for reuse, preferably for reuse in nature, agriculture, industry, urban development and groundwater level management, wherein the waste water from the sewer is purified in a treatment plant in which means are used to adjust the degree of the efficiency of nutrient removal by the treatment plant (preferably periodically), so that a predesired, variable nutrient content is achieved in the treated waste water from the sewer.

Description

Method for the decentralized production of product water from wastewater originating from sewers

The invention lies in the field of purification of waste water to a water quality suitable for reuse in nature, agriculture, industry, urban development and groundwater management, and in particular in the field of biological purification.

BACKGROUND

In recent years, large parts of the Netherlands have increasingly been confronted with prolonged periods of drought. At the same time, the irregular periods of increasingly extreme precipitation make it more difficult to retain rainwater. The recently published (August 2021) IPCC report shows that both drought and extreme precipitation will only increase due to climate change. This leads to conflicts regarding the claim to scarce groundwater between citizens, farmers and nature. Annual social costs as a result of drought are estimated in billions of euros in the Netherlands alone. Similar problems are also occurring internationally and, as a result of water scarcity, there is a need for water for irrigation, sprinkling, industrial activity and groundwater level management.

Treated wastewater is an always-available water source that can help alleviate drought problems. Whereas in the Netherlands the credo of retention, storage and reuse is central to rainwater, the opposite applies to wastewater. Due to the cost-effectiveness of large-scale and central sewage treatment plants (WWTPs), a national centralization of WWTPs has taken place. Waste water is transported from the location where it is produced to a central (often low-lying) sewage treatment plant. The final destination of the treated effluent is the sea. However, central effluent reuse is made more difficult, because the location where the water demand is on the one hand and the effluent is produced on the other (the sewage treatment plant) almost never coincide.

Narain-Ford shows in “Shifting the imbalance: Intentional reuse of Dutch sewage effluent in sub-surface irrigation” (Science of the Total Environment journal, 2021, Volume 752) that treated effluent from WWTPs can make a significant contribution to the water demand from Agriculture. Due to the central character of the wastewater structure, however, only less than 10% of the agricultural land can be reached from the existing 335 WWTPs, assuming a maximum distance of 2 km from the WWTP. Half of the agricultural land could be reached if this distance were 5 km, but that in turn entails much longer transport pipeline networks.

Furthermore, responding to fluctuating water demand (quantity) and quality to a “tailor-made water” demand is virtually impossible. Nature, farmers and industry demand different quality and quantity of water. The result: In the Netherlands, this potential water source is not used at any WWTP. There is therefore a need to resolve this mismatch in location, quality and quantity in the current paradigm; how can we supply customized water where we are flexible in location, quantity and quality.

The solution of sustainable and local water extraction can be found in urban, rural and industrial sewers. A quantity of required waste water can be extracted from this and upgraded to a quality required by the customer.

After all, such sewers collect and transport waste water from large inhabited areas and/or from various industrial dischargers and offer certainty of continuity of water supply, while there is also a greater degree of continuity in the composition of the waste water than is the case for individual dischargers. would be. The sewage system is also reasonably close at virtually every location of residential, agricultural and industrial activity. Furthermore, purification technology is available that can purify the wastewater to the composition required for use and this water extraction relieves the sewage or wastewater treatment plant that treats all the sewage water before it is discharged into surface water. Extraction of water for reuse from sewer systems is also referred to as “sewer mining”. It should be noted that the term “sewer mining” does not refer to the extraction of water by purifying waste water that is treated by WWTP at the end of the sewage system before being discharged into surface water. The term expressly refers to the decentralized collection and treatment of only part of the wastewater flowing through the sewer system to the sewage treatment plant. And this in the vicinity of the location where the thus "extracted" water is needed.

This is how Plevri describes in “Sewer Mining as A Basis for Technological, Business and Governance

Solutions for Water in the Circular Economy: The NextGen Athens Demo" (Environ. Sci. Proc. 2020, 2, 54) a method to provide an urban agriculture, park and business parks with suitable water via sewer mining. An

Membrane BioReactor (MBR: a combination of biological treatment and membrane filtration) followed by a UV disinfection with a fixed capacity of 25 m%h that upgrades the sewage water from the sewer to a fixed product water quality, which is then used for irrigation and in winter also for aquifer replenishment. Unfortunately, this method has several different economic and sustainability disadvantages. Firstly, it results in a product quality that is extremely suitable for aquifer replenishment in winter, but which, however, results in water quality for irrigation applications that is significantly better than necessary and desirable for irrigation applications. Since MBR is characterized by a relatively high energy consumption for both biological treatment and certainly also for membrane filtration, this is a sustainability disadvantage. Furthermore, the extensive purification means that the obtained irrigation water contains hardly any nutrients such as nitrogen, phosphate and organic substances, while they actually stimulate the growth of plants and vegetation.

This increases the use of artificial fertilizers and biostimulants, which then have to be added. For the production of fertilizer and biostimulant, Plevri uses an additional facility that can make such substances from organic waste via composting.

Makropoulos (Makropoulos et al, 2018, Journal of Environmental Management, 10.1016/j.jenvman.2017.07.026) describes how the operation and maintenance of decentralized wastewater treatment plants for sewer mining can be improved by embedding them in an ICT infrastructure, so that the status and operation of the wastewater treatment plants can be monitored remotely. Makropoulus uses the combination MembraneBioReactor (biological) followed by reverse osmosis (chemical/physical) to purify the wastewater to a constant water quality with minimal nutrients. However, as noted, the MBR consumes a relatively large amount of energy.

In the state of the art there is a continuous need for further implementation and optimization (including making waste water treatment more sustainable).

SUMMARY OF THE INVENTION

Surprisingly, the inventors have found experimentally that a more economical method of sewer mining is achieved by — using the same treatment plant — adjusting the level of the efficiency of the treatment of the wastewater in a controlled manner during the seasons. As a result, for example, during the growing season of crops and vegetation, the residual concentration of nutrients in the produced water can be kept as high as practically, agrotechnically desirable and legally permitted, so that there is no need for additional use of fertilizers and biostimulants to achieve a maximum growth yield, or at least the use of such resources is reduced. Outside the growing season, or during periods during the growing season when there is temporarily no need for additional irrigation or sprinkling, the degree of efficiency of the purification process can be increased to such an extent that the residual concentration of nutrients remains low. The control and controlled variation in nutrient removal also has applications elsewhere.

Preferably, the purified water with a low nutrient content is offered for use for non-agricultural applications, urban water elements, aquifer and surface water replenishment.

In the broadest sense, the invention therefore relates to a method for 'sewer mining', i.e. the (with the advantages of decentralized or local) purification of waste water from sewage (sewage) purifying waste water from sewage (sewage) for reuse, preferably for reuse in nature, agriculture, industry, urban development and groundwater level management, whereby the wastewater from the sewage system is treated in a treatment plant where means are used to adjust the degree of the efficiency of nutrient removal by the treatment plant (preferably periodically), so that a predesired, variable nutrient content in the treated waste water from the sewer is achieved.

In particular, the invention relates to a method for 'sewer mining', i.e. the purification (with the advantages of decentralized or local) of waste water from sewage (sewage) for use in the agricultural industry, for irrigation or for groundwater level management, whereby the waste water of the sewage is treated in a treatment plant where means are used to adjust the degree of the efficiency of the nutrient removal by the treatment device, with which the efficiency of the nutrient removal preferably during the growing season of crops and vegetation (temporarily and/or periodically} - or else when there is a need for this - is reduced, so that a predesired increased nutrient content is achieved in the treated waste water from the sewer.

The inventors have worked out that adjusting (regulating or controlling) the nutrient content can be achieved in a number of ways. This can preferably be done by adjusting the wastewater flow rate added to the purification reactor and/or adjusting the aeration duration and/or aeration intensity (preferably adjusting the aeration duration, parts of the aeration cycle and/or aeration intensity), but can, although with less preferably, can also be carried out instead or in combination with it by mixing the treated effluent with less-treated water from the sewer (the so-called bypass). Less purified water means that one or more of the purification steps have not been carried out. These are further elaborated below. However, the invention is not limited to these embodiments.

LIST OF FIGURES

Figures 1, 2, 3a, 3b and 4 schematically represent the process of sewage treatment, in which waste water is passed from the sewer 1 by means of a pump and flow meter 2 to a purification reactor 3, or a combination of successive purification reactors. The purification reactor supplies purified product water with nutrients 4, and the process provides means to control the concentration of nutrients at a desired (increased or decreased) level (dotted circle and line}. The circle 8 is a schematic representation of an embodiment where there is means have been added to measure the content of nutrients in the purified water These are now shown schematically after the treatment, but (not shown in the Figures) can also be integrated in the reactor(s), or (not in the

Figures shown:) before feeding to the reactor to determine the nutrient content in the water fed to the reactor(s). The dotted line indicates that there is a link between the determination of the content and means to bring the concentration of nutrients to a desired (increased or reduced) level. According to the invention, this process can be optimized, for example, by using a biological purification reactor 3A, where nutrient removal is regulated to a desired level by regulating the supply of waste water through the pump and flow meter 2 to the biological purification reactor 3A (Figure 1}, and/or or the duration of the aeration cycle of the flow in the biological purification reactor 3A (Figure 2).In another embodiment, a chemical/physical purification reactor 3B is used, and the chemical/physical process conditions in the reactor 3B are adjusted for a change in the nutrient concentration (Figure 3A) Instead, or in addition [not shown in the Figure], the process can be provided with a flow bypass 6, in which the degree of flow not first to the reactor 3B but directly with the effluent from the reactor 3B is mixed, is adjusted in response to a (desired) change in the nutrient concentration (Figure 3B). Figure 4 shows a combination of the method according to Figures 2 and 3A, with a cross-over connection 7 whereby at least part of the effluent from the biological purification reactor 3A can be supplied to the inlet of the chemical/physical purification reactor 3B.

DESCRIPTION METHOD AND EMBODIMENTS

The term "sewer mining" is known to those skilled in the art. |.t.t. the purification of waste water in a large water treatment plant or sewage treatment plant, usually at the end of a sewer network or at a node where several sewers meet, where the waste water is discharged, with sewer mining according to the invention, at least part of the water of a sewage decreased. With the inventors' insights, it is possible to use sewer mining to reuse processed water economically in nature, agriculture, industry, urban water elements and groundwater level management. Preferably, the water is taken from the sewer system and purified at a 'local' location that is less than 5 km, preferably less than 1 km, from the location where the processed water is reused. In one embodiment, a portion, preferably less than 80% of the sewage water that is sent to a

WWTP is fed, used in the method according to the invention. In the special case that the reprocessed water is required in the immediate vicinity of a WWTP, the invention can also be applied on the site or a site adjacent to the WWTP.

The idea of the invention is that the nutrient content (phosphates and nitrogen) — through variation in the efficiency of purification — in the purified water can be varied in a controlled manner and adjusted to the varying needs in nature, agriculture and industry. Various possible work and embodiments of the invention are presented below, all with the aim of regulating the nutrient concentration in the effluent of wastewater that is taken from the sewage system and treated, to a previously desired reduced or increased level.

Those skilled in the art will recognize that the described embodiments are alternatives and do not exclude other non-detailed embodiments which achieve the same effect, and that such non-detailed embodiments and similar methods are therefore part of the same invention.

Nutrients include - and preferably - nitrogen- and phosphate-containing compounds that are present in the waste water. Such compounds are suitable for use in agriculture. There are legal restrictions on the maximum amounts of phosphate and nitrogen-containing compounds that may be used in agriculture. These are determined annually and are also calculated annually, the so-called phosphate and nitrogen utilization space. Such phosphate application standards and nitrogen application standards determine how much phosphate and nitrogen may be used on agricultural land on an annual basis.

With the method according to the invention, differentiation can be made within that usage space, and thus easily optimized according to the need for nutrients, which also varies throughout the year. For example, there is a greater need for nutrients during the growing season than outside. It is noted that, in addition to nitrogen- and phosphate-containing compounds, other organic and inorganic compounds in the waste water also belong to the category of nutrients and that they also form part of the invention.

The invention does not relate to a specific form of sewage treatment, but can be used with any conventional wastewater treatment. However, it is preferable for the purification of the waste water that is taken from the sewage system to use one or more biological purification reactors, which means that the substrate is converted by micro-organisms (MO). This can be done through both anaerobic and aerobic technologies. In anaerobic technologies, the MO thrives without adding significant oxygen to the mixture of MO and wastewater. The MO mainly converts the contaminants into methane gas and growth of the MO. In aerobic technologies, oxygen is supplied to the MO in some way, for example by introducing air through point aerators, through bubble aerators, or otherwise. In particular, the aerobic MO converts organic carbon-containing contaminants into carbon dioxide and growth of MO.

If biological purification is used, aerobic water purification is preferred.

This can be done with conventional continuous activated sludge systems (using flocculated activated sludge}, or - preferably - with aerobic granular sludge (i.e. aerobic granular sludge water treatment), such as the

Nereda® technology. Scientifically, aerobic granular sludge is qualified as sludge with a size of at least 0.212 mm and typically smaller than 6 mm, where no coagulation takes place during settling, so that the sludge volume index (a measure of the settling properties of sludge) after settling for 5 minutes is comparable to which settle after 30 minutes. The main advantages are that the aerobic granular sludge settles much faster (4-40 m/h) than flocculation activated sludge (0.5 - 2 m/h) and that the above-mentioned anoxic and anaerobic conditions for nutrient removal are created within the granules; even if the granular sludge is aerated and the aerobic MO breaks down the organic, carbon-containing contaminants, denitrification can thus take place simultaneously deeper in the granule and phosphate is fixed. In comparison, where activated sludge systems often use a MO concentration between 3-5 g/l, this is 6-15 g/l for granular sludge systems. In biological purification according to the invention, granular sludge with a size between 0.212 and 6 mm is therefore preferably used, preferably with MO concentrations of 6-15 g/l. An example of granular sludge technology which is widely used in practice for the aerobic treatment of waste water and which is suitable for the method according to the invention is described in

WO2004/024638. The contents hereof are incorporated herein by reference in their entirety.

In other preferred variants use is made of one or more different types of biological purification reactor(s) that work with biofilms and/or algae and plants.

In aerobic biological treatment, the conditions can advantageously be set up in such a way that MO can also thrive, which are able to remove the nutrients from the wastewater.

Such process conditions often comprise so-called anaerobic and anoxic conditions. In anoxic conditions, MO can convert nitrate and nitrite to nitrogen gas and thus reduce the nutrient content. In the so-called anaerobic conditions, the concentration of such oxidized nitrogen compounds is limited or not present at all. Under such conditions, MOs that store phosphate can thrive, for example through the MO of the

PAO class (phosphate accumulating organisms). It should be noted that while the common term “anaerobic” of these process conditions is the same in anaerobic purification technologies, it is a completely different class of MO and should not be confused with the anaerobic purification technologies described above which typically produce methane.

Non-exhaustive examples of a biological implementation are shown in Figures 1 and 2.

Instead of biological purification, chemical/physical purification reactors can be used to purify the wastewater that is taken from the sewerage system, such as filtration, flotation, ion exchange, adsorption, precipitation, UV disinfection, oxidation or electro-coagulation/physical purification reactors. . The invention is not in the use of the degree of purification, and those skilled in the art are familiar with such chemical/physical purification reactors. An overview of such techniques can be found in general textbooks such as Metcalf & Eddy's WASTEWATER ENGINEERING: TREATMENT AND

RESOURCE RECOVERY (ISBN 978-0-07-340118-8). Non-exhaustive examples of such techniques are shown in Figures 3A and 3B.

In addition, combined embodiments are possible, in which at least part of the water that has been purified in a biological purification reactor is subjected to further purification in a chemical/physical reactor. An example of this is shown in Figure 4.

It is preferred that when treating sewage water at least aerobic granular sludge purification is used with granular sludge with dimensions between 0.212 and 6 mm, preferably with MO concentrations of 6-15 g/l. An example of this is the Nereda® technology.

Although the invention has been described herein with reference to purification in a reactor, the process can also be carried out with multiple reactors connected in series or in parallel.

In one embodiment, in the method according to the invention, the nutrient content is not determined or maintained on site or online, but the quality of the purification and thus the nutrient content is controlled at predetermined times. However, it is preferable that the nutrient content in the reactor(s) and/or in the effluent is measured continuously or periodically.

This can be done automatically or manually with instrumentation. By continuous or periodic monitoring of the nutrient content in the reactor(s) and/or in the effluent of the reactor(s), regulation can be carried out more accurately.

Preferably, the degree of nutrient removal is adjusted to the needs of the growing season. The growing season depends to some extent on the crops grown. In the Netherlands, the start of the growing season is generally defined as the day on which the average 24-hour temperature reaches 5°C and does not drop below that until 1 July. The growing season usually begins around April 1 and runs through the end of September. The advantage of the method according to the invention is that the supply of nutrient content can be adjusted at one's own discretion, to the growing season, weather influences, groundwater level, etc.

If needed, typically determined by the start of the growing season, the nutrient content in the treated wastewater can be increased by temporarily lowering the level of treatment. This can be done by increasing the waste water flow rate that is added to the purification reactor. In the case of biological purification, instead or in combination with it, the degree of purification can be reduced in a controlled manner by reducing the aeration duration and/or aeration intensity. In the first case the sludge concentration and the contact time between waste water and sludge (with MO) are temporarily reduced, in the second case the efficiency of conversion by the aerobic MO is reduced.

When there is a need to lower the nutrient content of the treated wastewater (preferably at the end of the growing season), the efficiency of the treatment can be increased. This can be done by reducing the waste water flow rate that is added to the purification reactor. In the case of a biological purification, instead or in combination with it, the degree of purification can be increased in a controlled manner by increasing the aeration duration or aeration intensity. In the first case the sludge concentration and the contact time between waste water and sludge (with MO) are temporarily increased, in the second case the efficiency of conversion by the aerobic MO is increased. In addition, it is also possible to vary in pre-denitrification and pre-aeration.

The increase or decrease of the nutrient content of the treated wastewater is done by an active action (described above) (manually or automatically controlled), and thus distinguishes itself from the normal fluctuations in the basic level of nutrients in the wastewater.

Preferably, said active action leads to a (desired) change in the concentration of nutrients (down or up) of at least 10%, more preferably at least 20%, even more preferably at least 30%, most preferably 30- 500%. The relative changes in TN and TP are added together. Preferably, the nitrogen and phosphate concentration during the growing season is between 10-15 mg/l for a predetermined time.

TN (total nitrogen) and 0.5-2 mg/l (preferably 1-2 mg') TP (total phosphorus) checked, and treated wastewater is checked for a second time to less than 5 mg/l outside the growing season TN and less than 0.5 mg/l TP. Less preferably, for countries where this falls within the legal requirements, the nitrogen and phosphate concentration during the growing season is for a first determined time between 25-50 mg/l TN (total nitrogen) and 2-15 mg/l TP (total phosphorus) is checked, and treated waste water outside the growing season is checked for a second time to less than 15 mg/l TN and less than 2 mg/l

TP. Within the legal frameworks, the nutrient content can also be actively increased at one's discretion outside the growing season, taking into account the annual nitrogen and phosphate usage allowances. In practice it is preferred that the first and/or second predetermined period of time is at least 1, preferably at least 2, more preferably at least 3 months. Within these period(s) there is preferably opportunity for further variation depending on the circumstances (change in demand, for example due to changes in the groundwater level or weather influences).

Irrespective of the choice of reactor(s) mentioned above, in one embodiment the degree of nutrient removal from the purification reactor(s) is adjusted by adjusting the waste water flow rate added to the purification reactor(s). This can be done in response to a predetermined, desired temporary increase or decrease in nutrients, and/or in response to an observed change in the nutrient concentration of the effluent. The degree of nutrient removal in the above mentioned biological and various chemical/physical purification reactors depends on the flow rate. For example, using the same purification reactor(s), for example during the growing season, when there is a need for water with maximum amount of nutrients, the water throughput to the reactor(s) can be increased, which reduces the purification efficiency for nutrients. The degree of water purification is matched experimentally with the desired purification efficiency, whereby the degree of desired purification efficiency is adjusted during the growing season, but also temporarily if there is no demand for irrigation/sprinkling. The advantage of this preferred variant is that a practically maximum possible quantity of purified water is obtained using the same purification reactor. Preferably, for example during the growing season, the flow rate fed to the reactor is changed by at least 10%, preferably at least 20%, more preferably at least 30%, and preferably periodically. During the growing season it concerns an increase compared to. the flow rate that is added to the reactor(s) outside the growth zone. This preferred variant is shown in figure 1 and the dotted line illustrates the adjustment of the flow rate based on growing season and nutrient concentration in the effluent.

Figure 2 shows another preferred variant in which the degree of nutrient removal from the biological purification reactor(s) is adjusted by adjusting the duration of the biological purification, as in the Nereda® reactors. The biological removal of nutrients in these (cyclically operated) reactors mainly takes place during the aeration cycle. The dotted line illustrates adjusting the duration and/or aeration cycle based on growing season and nutrient concentration in the product water. In this preferred variant, the degree of nutrient removal during this aeration cycle is monitored with instrumentation and, if a sufficient degree of removal has been achieved, the aeration cycle is aborted. In another variant, the moment of breaking off the aeration cycle in the Nereda® reactor is determined experimentally. Comparable to the above method, in variants using purification reactors based on activated sludge and/or biofilm, the duration or intensity of the aeration is adjusted to the desired degree of nutrient removal.

The term 'experimental' above means that the nutrient content can be measured and a change in the measured content can be reacted to and steered towards a desired value, but it is - less preferred - also possible to determine the adaptability in advance.

Preferably, for instance during the growing season, the aeration duration and/or intensity in the purification reactor(s) is changed by at least 20%, more preferably 30-60%. During the growth zone, this concerns a reduction in relation to the aeration duration and/or intensity that is applied to the reactor(s) outside the growth zone. Instead of or together with the change in the aeration time and/or intensity in the purification reactor(s), the pre-denitrification step can also be omitted or limited. The advantage of these embodiments is that changes in individual nutrient concentrations are controllable; the person skilled in the art is familiar with the optimum conditions for the various MO and can thus change the N content (and within this the NH4 and NO3 content) and P separately if desired.

This has been elaborated in examples 2 and 3, and serve as a guideline for the skilled person.

In one embodiment, the degree of nutrient removal can be adjusted by changing whether or not, or limiting, the pre-aeration of the water before it is fed to the purification device. To lower the phosphate concentration temporarily and in a controlled manner, a coagulant can also be added to capture phosphate.

These embodiments are elaborated in Example 3.

In yet another embodiment, the degree of nutrient removal can be adjusted by changing the application of optional ultrafiltration and an anion exchanger to the purified water from the purification reactor(s).

It is within the skill of the artisan to select or combine from the above embodiments as desired to adjust the degree of nutrient removal.

In other preferred variants that make use of chemical/physical purification reactors, the degree of nutrient removal is adjusted to the desired removal efficiency by adjusting the chemical/physical process conditions (Figure 3a), such as pH, redox potential, chemical dosage, filter rinses, or partial by- fit of one or more of the physico-chemical process steps (Figure 3b).

In another preferred variant, the purification takes place in two or more parallel purification reactors, in which the degree of nutrient removal of at least one line is adjusted to the agrotechnical degree that is desirable. This variant is illustrated in figure 4. The amount of nutrients after the biological purification reactor of street 1 is varied based on the growing season. Street 2, in figure 4 consisting of a combination of biological and chemical/physical purification reactors, produces a product water with a seasonally independent nutrient quality. This preferred variant has the advantage that at the same time one or more other purification reactors simultaneously and uninterruptedly supply a product water quality that meets specific further purification requirements, for example for use in industry, households or aquifer replenishment. To increase operational flexibility, the streets can also exchange partial flows via cross-over connections (6. In figure 4).

The expert will recognize that the forms described correspond with each other in that the degree of nutrient removal in the purification reactors is adjusted to the most desirable and/or legal maximum amount for the growth of the crops. The skilled person will also recognize that the described methods do not exclude variants, alternatives and other embodiments that achieve the same effect and that such embodiments and similar methods are part of the same invention.

Examples

The examples show that the nutrient concentration in the effluent can be controlled to a level desired for use by taking various measures and that by means of those measures the concentration of nutrients can be increased and/or decreased in a controlled manner. The skilled person will recognize that the examples and simulations presented are not exhaustive and that within the invention the desired nutrient concentration could also be adjusted in other ways.

Example 1: adjustment of nutrient content by adjustment of flow rate

As an aid to designing and optimizing purifications utilizing the invention, the inventors have developed a simulation model that predicts TN and TP. The simulation is based on a facility where wastewater is fed from the sewer system.

The facility is equipped with a buffer tank with stock of waste water and sludge remaining after purification. The sludge is then transported to the central sewage system as required.

The collected waste water is pre-treated in a grid/sieve filter installation to remove coarser particles and collected in an influent buffer of 35 m3. Subsequently, the waste water is pumped to 3 biological purification reactors that operate according to aerobic granular sludge technology (Nereda® technology). In the simulation, the reactors each have a capacity of approx. 50 m3, but of course this can also be used, and this is a model for other layouts using more or less number of reactors and reactors with different contents. The treated waste water flows from the reactors (upper pipe) to a downstream filter. Below this filter is space where pumps, instrumentation and electrical control are housed.

Outside the growing season, period 1 in table 1, a maximum nutrient concentration of < 5 mg/l nitrogen (TN in table 1) and < 0.5 mg/l phosphate (TP in table 1) is produced in a controlled manner, which quality is sufficient to replenish the groundwater level for 30 hectares of agricultural land. This period lasts about 4 months and coincides with winter. In this situation, the installation described above takes in approximately 200 m2/day of waste water from the sewer.

During the high tide of the growing season, period 3 in table 1, which coincides with the summer and lasts approximately 4 months, the nutrient concentration is controlled to a lower quality, whereby the nitrogen concentration is 15 mg/l TN and the phosphate concentration 0.5-2 mg/l TP. These concentrations meet the European requirements for use, but add value to the product water as the nutrients promote growth. The product water is used, in this example, to irrigate an agricultural plot twice a month with a dose of 20 mm. In the example, the reduced purification efficiency of nutrients is achieved by increasing the wastewater intake to 450 m3/day. With this, 110 hectares of product water can be supplied.

Prior to the peak of the growing season, when nutrient requirements are even more limited, the nutrient concentration is controlled to 10 mg/l TN and 0.5-2 mg/l TP by increasing the waste water intake to 400 m compared to winter */day. This means that 100 hectares can be supplied with 20 mm of product water once a month. This is period 2 in the table and lasts approximately 2 months.

Finally, in period 4, the nutrient concentration is checked to, in this example, a value that is equal to period 2. Also with this, 100 hectares can be irrigated.

The example clearly shows the advantage of the invention: by using the same installation and without the need for additional investments, the annual amount of water is increased from approx. 200 m3/day to an annual average of 350 m3/day. Moreover, assuming a typical nitrogen requirement for crops of approximately 100 kg N/ha, more than 5-10% is saved on nitrogen fertilizer.

Table 1: Maximum nutrient concentrations due to variation in flow rate

Period Duration (month) | Treated TN (mg/l) | TP (mg/l) | hectares

I er a

Example 2: adjustment of nutrient content by adjustment of aeration

Using the same Nereda installation as in example 1, it was also simulated how, compared to the reference, a desired change in the nutrient concentration of the produced water can also be achieved by adjusting the duration and/or intensity and/or composition of the aeration cycle. This is based on a fixed flow rate of 270 m3/d.

The results of the simulation are shown in table 2. In the reference (example 1) a total reactor cycle time of 360 minutes is used and the build-up of the aeration consists of a period of pre-dentrification, aeration with a “normal” intensity to a dissolved oxygen operating point of 2 mg/l and a post-denitrification period. Furthermore, no waiting time is used between subsequent reactor cycles.

Table 2: Maximum nutrient concentrations due to variation in aeration duration and/or intensity and pre-denitrification

E 8 o example 3 £ 5 55 £ = S S £ ee HE cd Essa |823E SE [4 ee [0 Than ee a [TT 2a Controlled | 240 Off Normal No | 0.5 12 1

Cee TR 2b Controlled | 240| On Low No | 10 4 1

CERT

2c Controlled | 240 On Low Yes 6.5 45 1 increment of

NH4 and NO3 5 In simulation 24 the cycle time has been shortened to 240 minutes and pre-dentrification has not been used in the construction of the aeration. Compared to the reference, this results in an increase in the maximum nitrate concentration from 4 to 12 mg/l and illustrates how a controlled increase in the nitrate concentration can be achieved.

In simulation 2b, the intensity of the aeration has been reduced by steering to an oxygen operating point of 1 mg/l. This illustrates how a controlled increase in nitrate concentration can be achieved.

In simulation 2c, the cycle time has been shortened to 240 minutes and a waiting time of 45 minutes has also been applied between successive complete reaction cycles. This illustrates how a controlled increase in nitrate and ammonium concentration can be achieved.

Example 3: adjustment of maximum P-nutrient content

Also, in the installation as described in examples 1 and 2, the concentration of phosphate in the produced water can be adjusted. This is again simulated with respect to the reference, assuming a fixed flow rate of 270 m3/d, a cycle time of 360 minutes, a normal aeration intensity, use of pre-denitrification and no waiting time between reactor cycles. The results of the simulation are shown in table 3.

Table 3: Nutrient concentrations due to variation in aeration duration and/or intensity example Controlled | NH4 NO3 P

TT lams opn eo a ee eee 3a Controlled | Shorten <0.5 <4 <2 me 3b Controlled | Pre-aeration <0.5 <4 <2

Clee 3c Controlled | Dosage <0.5 <4 <0.5

Clee owe 3d Controlled | Increase <0.5 <4 <0.5 ee

In simulation 3a, the duration of the reactor feed has been reduced from 60 to 30 minutes. As a result, less phosphate from the sewage water is absorbed by the MOs. In this simulation this results in an increase of the maximum phosphate concentration from <1 to <2 mg/l. This illustrates how a controlled increase of the maximum phosphate concentration can be achieved.

An increase in the phosphate concentration can also be achieved by applying a limited degree of pre-aeration of the sewage water before it is fed into the biological purification reactor. As a result, part of the fatty acids that are already present in the waste water are removed, so that less phosphate can be absorbed by the MOs in the biological purification reactor. In simulation 3b, pre-aeration has been applied in the influent buffer so that the dissolved oxygen concentration in the buffer is 1 mg/l and, as in simulation 3a, the desired increase in the phosphate content to <2 mg/l is thus obtained.

A controlled reduction of the phosphate concentration compared to the reference is also possible. In simulation 3c, coagulant is dosed during aeration, so that in addition to the biological removal of phosphate in the purification reactor, additional precipitation of iron phosphate also takes place in the reactor. The simulation shows that, when applying an iron coagulant dose corresponding to a molar ratio of 2 mol Fe per mol P in the waste water, the phosphate concentration is reduced to < 0.5 mg/l.

A desired controlled reduction of the phosphate concentration can also be achieved by increasing the fatty acid content of the sewage water. In simulation 3d this is achieved by hydrolysing the sewage water for 4 hours prior to biological treatment. Due to the increase in the fatty acid content, the MOs can absorb more phosphate during the purification process. In the simulation, the phosphate concentration is thus reduced to < 0.5 mg/l.

Example 4:

In another simulation, for the applicability of the upgraded water it is necessary that the suspended matter concentration is limited to < 3 mg/l and the nitrate concentration to < 2 mg/l N and the biological Nereda purification (3 reactors of each 50 m3 with the same cycle and flow rate as the reference of previous examples) followed by an ultrafiltration and an anion exchanger. The ultrafiltration reduces the suspended solids content to < 3 mg/l and the dissolved nitrate concentration in the ion exchanger is reduced from 4 mg/l N to 1 mg/l N.

Since the anion exchanger is not selective, the dissolved phosphate content is also reduced from 1 mg/l P after the Nereda purification to 0.1 mg/l P by the ion exchanger.

In the growing season, a phosphate content in the processed water of 0.5 mg/l P is desirable. In this period, the phosphate nutrient concentration of the upgraded water is increased by not purifying part of the effluent from the ultrafiltration through the anion exchanger, but by mixing this partial flow with the effluent of the anion exchanger, in other words, a partial flow is bypassed through the anion exchanger. . In this example, the by-pass is controlled to 33% of the ultrafiltration effluent flow rate. After mixing, the required quality of < 2 mg/l N is obtained and the phosphate concentration is 0.4 mg/l P. This phosphate concentration is lower than the desired concentration of 0.5 mg/l, but results in a factor 4 reduction of the use of phosphate fertilizer during the growing season.

Claims (15)

Conclusions 1. Method for purifying waste water from sewerage {sewage} for reuse, preferably for reuse in nature, agriculture, industry, urban development and groundwater level management, whereby at least part of the sewage water that is fed to a sewage treatment plant is taken and wherein the purification of waste water is decentralized, whereby the waste water is purified from the sewer in a treatment plant, where means are used to adjust the degree of the efficiency of nutrient removal by the treatment plant (preferably periodically), whereby a predesired, to be determined varying nutrient content in the treated wastewater from the sewer is achieved and where the nutrient includes inorganic compounds. 2. Method as claimed in any of the foregoing claim, wherein use is made of one or more biological purification reactor(s) for the purification of the waste water. 3. Method according to the preceding claims, in which for the biological purification of the waste water use is made of purification reactor(s) based on aerobic granular sludge, preferably with Nereda® technology. 4. A method according to any one of the preceding claims, wherein for the biological purification of the waste water use is made of purification reactor(s) that make use of activated sludge, biofilms, algae and/or plants. A method according to any one of the preceding claims, wherein the degree of nutrient removal from the biological purification reactor is adjusted by adjusting the wastewater flow rate added to the purification reactor. A method according to any one of the preceding claims, wherein the degree of nutrient removal from the biological purification reactor is adjusted by changing the aeration duration and/or aeration intensity. 7. A method according to any one of the preceding claims, wherein chemical/physical purification reactor(s) are used for the purification of the waste water, such as filtration, flotation, ion exchange, adsorption, precipitation, coagulation/flocculation, sedimentation, UV disinfection, oxidation or electro-coagulation, or by a combination of chemical/physical and biological purification reactors . A method according to any one of the preceding claims, for reuse in the agricultural industry, wherein by adjusting the degree of nutrient removal efficiency, the nutrient removal efficiency during the growing season of crops and vegetation is (temporarily and/or periodically) reduced, thereby a previously desired increased nutrient content in the treated waste water from the sewer is achieved. 9. A method according to any one of the preceding claims, wherein the degree of nutrient removal from the purification reactor is adjusted by changing whether or not, or to a limited extent, pre-denitrification and/or pre-aeration before it is added to the purification reactor, and/or in the whether or not, or limited use of a phosphate coagulant and/or ultrafiltration and an anion exchanger on the purified water. 10. A method according to any one of the preceding claims, wherein the degree of nutrient removal of the chemical/physical purification reactor is adjusted by changing the process conditions such as pH, redox potential, chemical dosage, regeneration/flushing interval, and/or partial bypass, etc. A method according to any one of the preceding claims, wherein the purification takes place in two or more parallel purification lines, the degree of nutrient removal of at least one line being adjusted during the growing season. A method according to any one of the preceding claims, wherein the nutrient content in the reactor(s) and/or in the effluent is measured continuously or periodically. A method according to any one of the preceding claims, wherein the adjustment changes the concentration of nutrients by at least 10%, more preferably at least 20%, even more preferably at least 30%. A method according to any one of the preceding claims, wherein the nitrogen and phosphate concentration, for a first determined time, preferably during the growing season, is between 10-15 mg/l TN (total nitrogen) and 0.5-2 mg/l (with preferably 1-2 mg') TP (total phosphorus) are monitored, and for a second specified time, preferably a period outside the growing season, wastewater treated to less than 5 mg/l TN and less than 0.5 mg/l TP is checked; or where the nitrogen and phosphate concentrations are monitored for a first time, preferably during the growing season, between 25-50 mg/l TN and 2-15 mg/l TP, and for a second time, preferably a period outside the growing season, wastewater treated to less than 15 mg/l TN and less than 2 mg/l TP is monitored. 15. Method according to any one of the preceding claims, wherein the water is taken from the sewage system and purified at a location less than 5 km, preferably less than 1 km, from the location where the reprocessed water is used and this distance is at least 20%. is shorter than the distance to a central sewage treatment plant.