Background of the invention

The invention relates to a membrane reactor for gas absorption .

Further, the invention relates to a wastewater treatment arrangement and to a method of recovering nitrogen from a waste stream .

More generally the solution relates to wastewater treatment engineering .

The obj ect of the invention is described in more detail in the preambles of independent claims of the application .

In present solutions rej ect waters are mainly conducted to a wastewater treatment plant or if the digester is at the wastewater treatment plant , the rej ect waters are recycled back to the beginning of the process where they cause an internal loading to the process . Biogas plants often pay an increased wastewater fee based on the nitrogen content of their wastewater and the treatment is quite costly . In some cases , this increased fee is not applied, and the water utility is not covering all the costs from the polluter . Nevertheless , the "polluter pays" principle can be expected to be implemented over time everywhere . Sometimes the rej ect water is also used as such as liquid nitrogen fertili zer . Since the rej ect water contains micropollutants , this solution is not for long-term either .

One known solution is a biological nitrogen removal processes wherein nitrogen compounds are transformed to nitrogen gas which escapes to the atmosphere . Ammonia stripping by air is also an established technology . However, it is not widely used despite being decades old invention, arguably due to high operational costs . Another process i s struvite precipitation, but it mainly recovers phosphorus not nitrogen and it requires biological phosphorus removal in the water process which is not widely used . There are al so membrane reactors developed for gas absorption . The membrane is a gas permeable membrane that allows ammonia to pass through . In the reactor two liquid streams (waste liquid where ammonia is dissolved as gas ) and acid stream are put in contact . Ammonia gas passes through the membrane and reacts with the acid . The membrane reactors have some benefits compared to the other mentioned solutions . However, it has been found that there are drawbacks in the current membrane reactors which limits their usage for treating different wastewaters .

Brief description of the invention

The idea of the invention is to provide a new and improved membrane reactor, wastewater treatment arrangement and method for gas absorption .

The characteristic features of the membrane reactor according to the invention are presented in the characteri zing part of the first independent device claim .

The characteristic features of the wastewater treatment arrangement according to the invention are presented in the characteri zing part of the second independent device claim .

The characteristic features of the method according to the invention are presented in the characteri zing part of the independent method claim .

The idea of the proposed solution is to provide a loosely packed membrane reactor for recovering ammonia from bulk liquid provided with ammonia concentration . Inside a tank of the membrane reactor are several membrane shapes which are loosely packed in relation to the si ze of the tank .

The membrane reactor comprises a tank inside which the bulk liquid to be treated is fed . Several hydrophobic gas permeable membrane shapes are arranged inside the tank and submerged in the bulk liquid . There are inlet means for feeding flow of ammonia stripping solution inside the membrane shapes , whereby the ammonia concentrated in the bul k liquid is allowed to pass through the membrane shapes and to react with the ammonia stripping solution . Further, total volume of the membrane shapes in relation to inner volume of the tank is 10 - 30 % whereby the membrane shapes are loosely packed inside the tank .

In the membrane reactor two liquid streams are put in contact so that the ammonia gas of the bulk liquid passes through the membrane and reacts with the ammonia stripping solution which may be acid .

The membrane reactor can also be called as a membrane contactor .

Some hydrophobic gas permeable membrane solutions exist on the market , but they are all based on tightly packed contactor design . This is not feasible for treating wastewaters , which is one focus of our invention, because the tightly packed contactors require high level of expensive pre-treatment to be able to treat water fractions with high solid concentrations .

In the disclosed membrane reactor, the membranes are loosely pos itioned in the reactor allowing use of high suspended solid concentrations .

Existing gas permeable membrane reactors do not allow any suspended solids in the treated water . This excludes their application for many waste streams . In the disclosed solution the membrane reactor can handle concentrations around 500 -700 mg/1 .

The hydrophobic gas permeable membranes are positioned in the membrane reactor differently . This allows treating a multitude of different waste streams without highly speciali zed and expensive pre-treatments like microfiltration .

A user, such as a biogas plant , can recover nitrogen from waste streams , such as digester rej ect water, without any excessive and costly pre-treatment to remove suspended solids . Nowadays the rej ect water treatment cost is a significant cost for the users , but the disclosed recovery process allows to avoid these treatment costs and makes it able to sell the recovered nitrogen . In some of the cases the disclosed solution is economically feasible even when only the avoided treatment costs are counted .

A further advantage is that compared to some other waste liquid treating solutions , the disclosed solution has a benefit of recovering nitrogen .

The disclosed solution has multiple benefits compared to tightly packed contactors :

- Low energy consumption

- High tolerance for suspended solids

- Membranes are not prone to fouling, scaling, or wetting

- Easy retrofitting into existing systems

- Lower consumption of resources because the process requires less pre-treatment wherein chemicals and energy are needed

According to an embodiment , each membrane shape comprises a space limited by surfaces of the membrane material and allowing the ammonia stripping solution to flow through the space .

According to an embodiment , total surface area of the membrane shapes in relation to inner volume of the tank is 20 - 100 m ² per 1 m ³ .

According to an embodiment , the membrane reactor is configured to treat bul k liquid provided with suspended solid concentration 500 -700 mg/1 .

According to an embodiment , the membrane shapes are membrane fibers .

According to an embodiment , the membrane shapes are hollow fibers . The ammonia stripping solution is fed through the hollow fibers . Thereby the membrane reactor can be called as a hollow-fiber membrane contactor .

According to an embodiment , the membrane shapes are membrane fibers and several membrane fibers are bundled to form membrane modules . Further, each membrane module is provided with an inlet for feeding the ammonia stripping solution flow into the membrane fibers of the bundled configuration .

According to an embodiment , the membrane reactor comprises several bundled modules . Number of the membrane fibers in one bundle may be 100 or more .

According to an embodiment , the membrane reactor comprises several membrane modules . Number of membrane modules may be six or more .

According to an embodiment , the membrane shapes are membrane plates . The membrane plates may be arranged so that fluid spaces are formed between them .

According to an embodiment , the membrane reactor comprises several membrane modules each of them provided with several membrane shapes . The membrane modules may be removable pieces whereby the membrane modules can be easi ly removed for washing and service . The removable membrane shapes are also easy and quick to substitute with new or serviced membrane modules .

According to an embodiment , the membrane module comprises a housing shell for protecting the membrane module . The housing shell is provided with openings to allow bul k liquid to flow through the membrane module .

According to an embodiment, flow of the bulk liquid inside the tank is made turbulent to ensure that the ammonia transfer efficiency is as high as possible . By means of the turbulence a laminar flow layer in the surface of the membranes is kept minimum . Thus , the turbulence in the flow improves efficiency of the gas penetration through surfaces of the membrane shapes .

According to an embodiment , the membrane reactor is provided with a bulk liquid mixing feature . The mixing is for providing good contact between the bulk l iquid and the membrane surface . According to an embodiment , the membrane reactor comprises a mixer for generating turbulence for the flow of the bulk liquid inside the tank .

According to an embodiment , the bulk liquid is kept in movement inside the tank so that the ammonia transfer stays efficient .

According to an embodiment , the solution comprises a hyperbolic mixer .

According to an embodiment , mixing regime in the contactor is designed to keep the laminar flow layer in the surface of the membranes to minimum to ensure the ammonia transfer efficiency is as high as possible . This can be done with hyperbolic mixer, but other solutions are apt for it as well , if the flow in the contactor is turbulent enough .

According to an embodiment , the tank is at atmospheric pressure , whereby the bulk liquid is without pressuri zation against the membrane shapes .

According to an embodiment , the bul k liquid to be treated is wastewater .

According to an embodiment , the used ammonia stripping solution is 0 . 5 mol - 1 mol/1 inorganic acid . The inorganic acid can be sulphuric acid, nitric acid, or phosphoric acid, for example .

According to an embodiment , the bulk liquid is urine , ammonia rich liquid, or waste liquid where ammonia is dissolved as gas .

According to an embodiment , the disclosed solution relates also to a wastewater treatment arrangement for treating wastewater wherein ammonia is dissolved as gas . The arrangement may be in connection with a wastewater treatment plant , for example . The arrangement comprises a membrane reactor for setting flow of the wastewater in contact with flow of ammonia stripping solution for ammonia recovery . The membrane reactor is in accordance with the features disclosed in this document and is configured to allow the ammonia gas to pass through several membrane shapes inside the membrane reactor and to react with the ammonia stripping solution circulated inside the membrane shapes and producing thereby ammonia salt .

According to an embodiment , the arrangement comprises a storage tank for the ammonia striping solution . The arrangement is configured to circulate the ammonia stripping solution between the storage tank and the membrane shapes until the ammonia stripping solution has been exhausted of the available binding sites for ammonia .

According to an embodiment , the disclosed solution relates also to a method for recovering ammonia from bul k liquid provided with ammonia concentration . The method comprises : feeding flow of the bul k liquid into a tank of a membrane reactor ; circulating flow of ammonia stripping solution inside several hydrophobic gas permeable membrane shapes arranged inside the tank and submerged in the bul k liquid; allowing the ammonia concentrated in the bulk liquid to pass through the membrane shapes and to react with the ammonia stripping solution and producing ammonia salt ; and providing turbulent flow of the bulk liquid inside the tank to prevent formation of laminar flow layer on surfaces of the membrane shapes . The method further comprises using a membrane reactor wherein the membrane shapes are loosely packed inside the tank and feeding to the tank bulk liquid with high solid concentration .

According to an embodiment , the method comprises treating unfiltered bulk liquid in the membrane reactor .

According to an embodiment , the method comprises treating bulk liquid with suspended solid concentration up to 700 mg/1 .

According to an embodiment , the method comprises using inorganic acids as the ammonia stripping solution .

According to an embodiment , the method comprises keeping the bulk liquid inside the tank at atmospheric pressure . According to an embodiment , the method comprises providing a batch of the ammonia stripping solution to a storage tank and circulating the ammonia stripping solution from the storage tank through the membrane shapes until the batch of the ammonia stripping solution has been exhausted of the available binding sites for ammonia .

According to an embodiment , the method comprises implementing partial pressure difference in ammonia gas transfer through the membrane shapes ; and maintaining the partial pressure difference by circulating the ammonia stripping solution inside the membrane shapes .

According to an embodiment , the method comprises treating the bulk liquid which is wastewater . The wastewater may be derived from different sources , such as from wastewater treatment plants , biogas plants , waste di sposal sites , etc .

According to an embodiment , the solution is retrofittable to existing systems .

The above-presented embodiments and the features they contain may be combined to provide desired configurations .

Brief description of the figures

Some embodiments of the proposed solution are illustrated more specifically in the following figures , in which

Figure 1 is a schematical top view of a membrane reactor,

Figure 2 is a schematical side view of one end part of a membrane module , and

Figure 3 is a schematical diagram showing conf iguration and fluid flows of a plant implementing the disclosed solution .

For clarity purposes , some embodiments of the proposed solutions are illustrated in a simplified form in the figures . The same reference numbers are used in the figures to denote the same elements and features . Detailed description of some embodiments

Figure 1 discloses an example of a loosely packed membrane reactor 1 . The membrane reactor 1 comprises a tank

2 inside which is arranged several membrane shapes 3 . The membrane shapes 3 are in Figure 1 membrane fibers 4 with hollow fiber tube configuration, as can be seen . Several membrane fibers 4 are bundled to form membrane modules 5 . Each membrane module 5 is provided with an inlet (not shown) for feeding ammonia stripping solution flow into the membrane fibers 4 of the bundled configuration .

The membrane modules 5 may be removable pieces whereby the membrane modules 5 can be easily removed for washing and service . The membrane module 5 may comprise a housing shell 6 for protecting the membrane module 5 . The housing shell 6 is provided with openings to allow bulk liquid to flow through the membrane module 5 .

As can be seen in Figure 1 , there is a lot of free volume inside the tank 1 . The membrane modules 5 are arranged in a loose manner and al so inside the membrane modules 5 there is free volume . Then bulk liquid with high solid concentration can be treated effectively .

The membrane reactor 1 may comprise a mixer 7 for moving the bulk l iquid inside the tank 2 and making turbulence therein .

The mixer 7 may be a hyperbolic mixer that creates radial flow and keeps bulk liquid in movement so that ammonia transfer in the membrane shapes 3 stays efficient . Other mixing arrangements are also possible .

Figure 1 further discloses that the membrane fibres 4 are arranged to form bundles 8 . Only two bundles are shown and the rest of them are indicated only by circles with broken lines . The bundles 8 may be surrounded by the housing shell 6 . Each bundle 8 may comprise 100 or more membrane fibers 4 and approximately 20 -50 % of the membrane module volume is taken by membrane fibers 4 . The membrane shapes

3 can be attached to the housing shell 6 by gluing, for example . The ammonia stripping solution flows into the module through one inlet and is distributed equally for all the membrane fibers 4 .

Figure 2 is a s impli fied view of one end of a membrane module 5 . At the end there are connecting elements 17 for connecting membrane shapes 3 to circulation system of ammonia stripping solution . An oppos ite end of the membrane module 5 also comprises connecting elements .

Figure 3 discloses a simplified schematic for hydraulic flows of a membrane reactor 1 . Ammonia stripping solution feed flow 9 is conducted from a storage tank 10 , or an acid tank, into a membrane module 5 and returning flow 11 exiting the membrane module 5 is conducted to the acid tank 10 . Bulk liquid flow 12 is fed from a feed tank 13 and is inj ected to a bottom of a tank 2 of the membrane reactor 1 . The bulk liquid exits freely from a top of the tank via a channel 14 . The schematic shows the effluent returning to the feed tank 13 , but it is case specific where it is directed . The ammonia stripping solution is circulated in the system by means of a first pump 15 , and the bulk liquid is pumped from the feed tank 13 by means of a second pump 16 .

Conventional membrane reactors ( or contactors ) are tightly packed . The disclosed membrane reactor is designed to have the membrane fibers or plates loosely packed . The modules can be submerged in a well-mixed container, presented in Figure 1 . The contactor is fed calmly, and it i s at atmospheric pressure .

The modules are submerged in liquid and can be changed at will . A schematic of the module is presented in Figure 2 . Each module contains hydrophobic gas permeable membrane shapes , which are for example ePTFE or teflon . The membrane packing density in each module is between 20 and 50 % (volume to volume ratio ) and packing density for the entire contactor is between 10 and 30 % (volume to volume ratio ) . In terms of surface area to reactor volume this i s between 20 and 100 m2 / m3 .

An example

Inside a circular membrane reactor tank (diameter 750 mm, height 1300 mm) i s six membrane modules . Each membrane module has inside diameter of 200 mm and height of 1000 mm and contains 150 membrane fibers . The membrane modules have large holes (diameter ~ 10 cm) on their outer shells to allow bulk liquid to flow through the membrane module while protecting the membrane fibers from excess ive mechanical stress . Liquid capacity of the tank is 320 litres when the membrane shapes are in use and 530 litres when empty .

Each membrane module is fed with ammonia stripping solution through an inlet below the membrane module . The ammonia stripping solution enters a space which distributes the flow equally to all membrane shapes inside the membrane module . Similar structure exists at the top of the membrane module where the flow exits the membrane module and is recirculated back to the storage tank . All support materials and housing shells are made of PVC-U material .

In the disclosed solution the bulk liquid is not pressuri zed against the membranes . A simplified schematic is show in Figure 3 , showing how flow directions for the bulk liquid and the ammonia stripping solution . The ammonia gas recovery is based on partial pressure ( concentration) difference . The ammonia transfers through the membrane passively . The partial pressure difference is maintained by circulating the stripping solution inside the membrane shapes and the ammonia stripping solution reacts with the ammonia, producing ammonia salt and rendering the ammonia partial pressure inside the membrane shape effectively to zero . Once the stripping solution has been exhausted of the available binding sites for ammonia, it will be replaced with new solution. The ammonia stripping solution is 0.5 - 1 mol/1 inorganic acid. Typical inorganic stripping solutions are for example, sulphuric acid, nitric acid, and phosphoric acid.

Mixing regime in the membrane reactor is designed to keep laminar flow layer on surfaces of the membrane shapes to minimum to ensure that the ammonia transfer efficiency is as high as possible. This can be done with hyperbolic mixer disclosed in Figure 1, but other solutions are apt for it as well, as long as the flow in the contactor is turbulent enough.

Overall, the ammonia transfer rate is strongly affected by ammonia concentration in bulk liquid and mixing regime. Depending on the quality of ammonia rich solutions, the ammonia transfer is within the range of 10-3 and 10-5 m/h in the material utilized. The membrane allows only ammonia to pass with few exceptions, ensuring that the resulting ammonia salt is pure. A wide range harmful substance analysis including 20 metals, 22 pharmaceuticals and 16 PHA16 compounds was performed on municipal wastewater reject water and the process products. Only metal traces detected in the aqueous salt were calcium and iron (~6 mg/1) , both of which were abundant in the bulk liquid. Similarly, detected pharmaceuticals were 5-methylbenzotriazol , Benzotriazole, Caffeine, Sertraline and norsertraline in low concentrations (0.1-3 pg/1) . No significant PAH16 concentrations were found. The membrane shape used in the process separates harmful substances with high efficiency with few exceptions .

The aqueous ammonia salt is a pure product itself. However, its value can be increased by concentrating it further or drying it to crystalline phase. The concentration and/or drying can be conducted by means which are commercially available (e.g., nanofiltration or thermal evaporation) and thus excluded from this patent description. The membrane reactor may be designed for easy maintenance . Each membrane module can be removed individually from the contactor or membrane reactor and washed and/or repaired if needed . Wash of the membrane shapes is simple and require water and diluted acid ( for example 0 . 1 mol / 1 sulphuric acid) to remove fouling accumulated on the membrane shapes . This can be also conducted by flushing the contactor or membrane reactor with appropriate liquid while the membrane modules are still ins ide for a time between 1 and 24 hours .

Furthermore , initial tests with various waste streams ( landf ill leachate , urine and rej ect water) showed that membrane hydrophobicity los s (wetting) is insignificant , although this can be regenerated by stronger chemical , like acids or hydrogen peroxide . In order to save in chemical consumption, the regeneration is better to be conducted for the membrane modules separately : the modules are soaked into regeneration liquid and the effect can be reinforced with UV-light treatment . After a time between 1 and 6 hours the modules are removed from the liquid and dried .

The figures and their description are only intended to illustrate the inventive idea . However, the scope of protection of the invention is defined in the claims of the application .