FIELD

The application relates to a method defined in claim 1 and an apparatus defined in claim 13 for separating solids from a liquid waste stream . Further, the application relates to a use of the method defined in claim 20 .

BACKGROUND

At the moment rej ect waters are mainly conducted to a wastewater treatment plant or if the digester is used 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 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 . In these cases , there' s no need to remove solids from the water . I f more advanced recovery processes are applied, solids need to be removed first .

At the moment there are no cost-effective method for removal of solids . This has been the weak point of many processes including stripping towers and biological processes using Anammox . The rej ect water typically contains solids that are difficult to remove . In the tests , both flotation and microsand ballasted sedimentation worked efficiently, but the costs of these processes were higher . Flotation and microsand processes are supplied by many different companies such as Veolia and Econet Group . For example , Actiflo process (US 8419947 Bl WATER TREATMENT METHOD COMPRI S ING A RAPID SETTLING STEP FOLLOWED BY A FILTRATION STEP DIRECTLY ON MICRO- OR ULTRA-FILTRATION MEMBRANES , AND CORRESPONDING DEVICE ) utili zed sand as a ballast to improve settling rate .

SUMMARY

The method, apparatus and use are characteri zed by what are presented in the claims .

In the method and apparatus for separating solids from a liquid waste stream, the liquid waste stream is treated by a ballasted sedimentation using a waste product of lime production as a ballast such that the liquid waste stream is mixed in at least one mixing step and sol ids are collected by a settl ing from the liquid waste stream, and a sol id stream comprising the solids is removed .

The proposed solution is a new type of solid separation process . The process is based on a ballasted sedimentation . In known processes a microsand is typically used as a ballast . In the present process a waste product of lime production, such as lime dust (LD) , can be used instead of microsand . The lime dust (LD) has several advantages . For example , it increases the pH of the treated stream, e . g . water, and thus improves the destabili zation of the colloids . It also improves the soil amendment qualities of the sludge where the solids and phosphorus are collected . The lime dust (LD) can be also removed with the solid stream, e . g . sludge , simplifying the process considerably . Other ballasted sedimentation microsand processes exist but the microsand is an inert material and has no benefits towards the final product like the lime dust (LD) . BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing, which is included to provide further understanding of the invention and constitute a part of this specification, illustrates some embodiments of the invention and together with the description helps to explain the principles of the invention . In the drawing :

Fig . 1 presents a flow chart illustration of a treatment process according to one embodiment .

DETAILED DESCRIPTION

The method for separating solids from a liquid waste stream comprises treating the liquid waste stream by a ballasted sedimentation using a waste product of lime production as a ballast such that the liquid waste stream i s mixed in at least one mixing step and sol ids are collected by a settling from the liquid waste stream, and removing a solid stream comprising the solids .

The apparatus for separating solids from a liquid waste stream comprises at least one mixing device for mixing the liquid waste stream and at least one settling apparatus for collecting solids by a settling from the liquid waste stream in order to treat the liquid waste stream by a ballasted sedimentation using a waste product of lime production as a ballast . Further, the apparatus comprises at least one means for removing a solid stream comprising the solids from the apparatus .

In this context , the liquid waste stream means any waste stream which is in a liquid form, e . g . a rej ect waste , rej ect water, wastewater, other liquid waste , or their combinations .

In this context , the settling means any settling, e . g . settling, sedimentation, clarification or comibinat ions of thereof or the like . The settl ing can be carried out in the settling apparatus, e.g. settler, sedimentation device, sedimentation tank or the like.

In this context, the solid stream may comprise one or more components, and at least one solid component, such as solids. In an embodiment, the solid stream comprises sludge or liquid, e.g. water. In an embodiment, the solid stream has a solid content which is over 30 % by weight, or over 40 % by weight, or over 50 % by weight. In an embodiment, the solid stream comprises suspended solids.

The apparatus comprises at least one mixing device for mixing the liquid waste stream, and the liquid waste stream is mixed in at least one mixing step. In an embodiment, the liquid waste stream is mixed in two or more mixing steps. In an embodiment, the liquid waste stream is mixed in a fast mixing step and in a slow mixing step. In an embodiment, the apparatus comprises at least two mixing devices or two or more mixing devices .

In an embodiment, the method comprises at least three treatment steps which are a fast mixing step, a slow mixing step and a settling step. In an embodiment, the method comprises four or more treatment steps. In an embodiment, the apparatus comprises at least a fast mixing device, a slow mixing device and the settling apparatus .

In an embodiment, a coagulation and/or a flocculation is carried out in the treatment steps.

In an embodiment, dewatering or drying is carried out after the settling. In an embodiment, dewatering or drying is carried out by using a filter or pressing device, preferably for dewatering or drying the solid stream and/or for separating nitrogen from the solid stream. In an embodiment, the apparatus comprises at least one filter or pressing device for dewatering or drying the solid stream and/or for separating nitrogen from the solid stream. In an embodiment, the apparatus comprises at least one filter, which is selected from a bag filter, belt press or pressure filter, for drying the solid stream and/or for separating nitrogen from the solid stream.

In an embodiment, at least a part of the solid stream is recirculated to a feed of the liquid waste stream, e.g. to a feed tank. In an embodiment, the apparatus comprises at least one recirculation device for recirculating at least a part of the solid stream from the filter or pressing device to a feed of the liquid waste stream, e.g. to a feed tank.

In an embodiment, the waste product of lime production is lime dust (LD) . Lime dust aided ballasted sedimentation is an efficient method to remove solids, such as suspended solids, from the liquid waste streams.

In an embodiment, the ballast is added in the fast mixing step and/or the slow mixing step. In an embodiment, the ballast is added in the fast mixing step or in the slow mixing step. In an embodiment, the ballast is added in the fast mixing step and in the slow mixing step. In an embodiment, the ballast is added after the slow mixing step, e.g. between the slow mixing and the settling. In an embodiment, the apparatus comprises at least one feeding device for adding the ballast. In an embodiment, the apparatus comprises at least one feeding device for adding the ballast to a fast mixing device and/or a slow mixing device, such as to a fast mixing device or a slow mixing device or to a fast mixing device and a slow mixing device. In an embodiment, the feeding device is arranged to add the ballast after the slow mixing device, e.g. between the slow mixing device and the settling apparatus.

In an embodiment, the ballast chemical, such as LD, in a liquid suspension, is injected to either a fast or slow mixing phase with a pump. Typical flow for LD is 3-15 % of the treated liquid waste stream. The LD particles attach themselves to the flocs, making them heavier, improving settling rate significantly. This allows for more compact settlers, thus making the economic costs for the installation lower than a conventional settler for similar wastewater flow.

In an embodiment, the process developed with NPHarvest uses a conventional coagulation-flocculation process utilizing fast and slow mixing phases and the settling. An economic estimation for this process is published in Uzkurt Kaljunen et al. (2021) , "J. Uzkurt Kaljunen, R.A. Al-Juboori, A. Mikola, I. Righetto, I. Konola, Newly developed membrane contactor-based N and P recovery process: pilot-scale field experiments and cost analysis, J. Clean. Prod., 281 (2021) , p. 125288".

In an embodiment, a pH control agent, a polymer and/or a coagulant is added in a fast mixing step. In an embodiment, a pH control agent, a polymer and a coagulant is added in the fast mixing step. In an embodiment, a pH control agent, a polymer or a coagulant is added in the fast mixing step. In an embodiment, the apparatus comprises at least one feeding device for adding a pH control agent, a polymer and/or a coagulant to a fast mixing device. The pH control agent, polymer and coagulant can be selected such that they are suitable for the liquid waste stream, e.g. base agent, sodium hydroxide or calcium hydroxide as the pH control agent, plastic polymer or organic polymer as the polymer, and/or starch or another coagulant as the coagulant.

In an embodiment, the liquid waste stream is injected with a liquid coagulant chemical using a pump producing a flow, e.g. that can be approximately 1 % of the feed flow. Typical concentration of the coagulant in the treated liquid waste stream may be between 0.1 and 3 g/1, depending on the characteristics of the treated waste stream. The chemical destabilizes the colloidal particles and bind them together in the fast mixing phase. The retention time in the fast mixing can be between 1 and 10 minutes and mixing efficiency is relatively high, up to 200 RPM. The destabilization by coagulation can be completed with either combining plastic polymer and metals or using organic polymers. The exact chemicals or their combinations are specific to each wastewater. For example, starch can be used in the process to precipitate solids (Righetto et al., 2021) , "Righetto, I., Al-Juboori, R.A., Kaljunen, J.U., Mikola, A., 2021, Wastewater treatment with starch-based coagulants for nutrient recovery purposes: Testing on lab and pilot scales, Journal of Environmental Management 284.

In an embodiment, pH is increased with calcium hydroxide or sodium hydroxide that is injected with a pump to a fast mixing phase. A flow is controlled by a controller aiming to adjust pH to range of 10-12. This improves the flocculation process and hygienic quality by deactivating bacteria in the stream (Konola, 2019) , "Konola, I., 2019, Optimization and quality assessment of fertilizers based on resource recovery technologies, Aalto University". For instance, active E. coll, le- gionella qPCR genus level DNA and RNA were reduced from 400 CFU/ml, 1800 GC/ml and 5.2xl0 ⁶ , respectively, to not detected. Also, sulphite reducing Clostridia spores and vegetative cell concentrations were considerably reduced but only by 50 and 80 % , respectively. Flocculation can take place in a slow mixing, where the hydraulic retention time is between 0.5 and 5 hours and mixing efficiency is considerably lower to avoid floc breakage, between 10 and 30 RPM.

In an embodiment, the solid stream is a phosphorus rich sludge. In an embodiment, the solid stream is dried, and a dried phosphorus rich product is recovered . In an embodiment , a nitrogen rich stream is recovered from the solid stream.

In an embodiment , at least a part of the nitrogen rich stream is recirculated to a feed of the liquid waste stream . In an embodiment , the apparatus comprises at least one recirculation device for recirculating at least a part of a nitrogen rich stream from the filter or pressing device to a feed of the liquid waste stream.

In an embodiment , a settler underflow, also called hygieni zed sludge , is dried further to capture valuable nitrogen in the l iquid phase and increase the solid content of the solid stream, such as sludge . The sludge can be pumped into a filter, such as a canvas fi lter bag, and filtered gravitationally, or it can be treated with more specialised equipment, such as a belt press or pressure filter . Extracted water with high nitrogen concentration is recollected and guided back to the process , such as a proces s feed tank . Depending on the characteristics of the dried sludge , it can be utili zed or further processed to enable efficient phosphorus recirculation .

In an embodiment , typical process removal efficiency can be 80 % for suspended solids and 80 - 90% for phosphorus with minor ammonia loss during the process . These ammonia losses are related to a pilot scale equipment and are expected to be negl igible in a full scale process . The process adds total solid content without affecting the volatile solid content of the sludge ( Sah, 2019 , "Sah, R . , 2019 , Development and optimi zation of pilot plant for nutrient recovery from rej ect water, Aalto University" ) in the form of inert calcium . The process deactivates bacteria (E . coli , salmonella and legionella) but does not entirely remove sulphite reducing Clostridia vegetative cells or spores . With case like Viikinmaki WWTP the amount of organic pollutants , heavy metals and pharmaceuticals was not high . Still , for example sertraline and norsertraline, diclofenac and carbamazepine concentrations were reduced, along with most PAHxs substances, such as benzo (b) fluoranthene and phenanthrene, (Konola, 2019) .

In an embodiment, the method or apparatus is used in a continues process.

In one embodiment, the method and apparatus can be used in a wastewater treatment, solid separation process, phosphorus recovery process, or their combinations .

It is to be understood that the embodiments described above may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention.

The method, apparatus and products described above have many advantages compared to previously known methods and apparatuses.

The present solid separation technique utilizes lime production waste flow to improve settling and drying properties of the resulting solid stream, e.g. sludge, while recovering phosphorus. The solid stream has soil amending qualities in addition to being hygienic and phosphorus rich.

Removal of solids is challenging from many known biological processes. By means of the present method and apparatus solids can be removed effectively. For example, the present solution is developed for a digester reject, but the present process can have applications widely for different liquid waste streams, e.g. waters. For example, using a lime dust (LD) ballasted sedimentation allows for small reactor size and thus lower price of reactors. It also improves the characteristics of the produced solid stream, such as sludge, and enhances the recycling of nutrients. The users, e.g. the biogas plant, can apply the present process as a pre-treatment for any kind of reject water treatment or it can be combined with the membrane reactor that recovers nitrogen . At the moment , the rej ect water treatment cost is significant for the plants , but the present recovery process allows to avoid these treatment costs and the plants also be able to sell the recovered nitrogen . For example , the present solution is economically feasible , when only the avoided treatment costs are taken into account in most of the cases .

The lime dust also improves the quality of the precipitate . The resulting sludge is easy to dry (Konola, 2019 ) and has a positive effect on soil should it be used in agriculture . Phosphorus can be bound on the solid precipitate , calcium and other metals . This makes the phosphorus slow to release for plants but abundant availability of calcium carbonate acts in similar way as gypsum and prevents excess phosphorus from leaching off from agricultural soils to adj acent water bodies .

EXAMPLES

The description provided below in connection with the appended drawing and examples are intended as descriptions of examples and are not intended to represent the only forms in which the examples may be constructed or utili zed . However, the same or equivalent functions and structures may be accomplished by different examples .

Fig . 1 presents a graphical description for the process to separate solids from a liquid waste stream ( 1 ) . The process is based on a solid separation process using lime dust ballasted sedimentation, e . g . in a wastewater treatment .

In the process of Fig . 1 , a waste product of lime production, such as lime dust (LD) , is used as a ballast ( 13 ) . An example of this material is Nordkalk' s LKD TY or MKD . The process of Fig. 1 comprises a fast mixing device (2) and a slow mixing device (3) for mixing the liquid waste stream (1) and a settling apparatus (4) for collecting solids by a settling from the liquid waste stream. A solid stream (6) comprising the solids and a liquid phase (5) are removed from the settling apparatus. The solid stream (6) is a phosphorus rich sludge. Further, the process comprises a filter (7) , e.g. bag filter, for drying the solid stream and for separating nitrogen from the solid stream.

In the process of Fig. 1, coagulation chemicals (10) are fed to a fast mixing, which is performed in the fast mixing device (2) , and the lime dust, LD, (13) is fed to a fast mixing in the fas mixing device (2) or to a slow mixing in the slow mixing device (3) , in one embodiment to the slow mixing, to make the flocs heavier and settle faster in the settling phase in the settling apparatus (4) . Resulting sludge has improved soil amending qualities and slowly dissolving phosphorus for agricultural purposes. Further, an organic polymer (11) and/or a base pH control agent (12) can be fed to the fast mixing device (2) .

In the process of Fig. 1, at least a part of the solid stream is recirculated from the filter (7) to a feed of the liquid waste stream (1) , e.g. to a feed tank, by a recirculation device. The phosphorus rich solid stream (6) is dried in the filter (7) , and a dried phosphorus rich sludge product (8) is recovered. Further, a nitrogen rich liquid stream (9) is recovered from the solid stream in the filter, and the nitrogen rich stream (9) is recirculated to the feed of the liquid waste stream ( 1 ) .

In one example, a wastewater treatment plant digester reject water can be treated by injecting PAX XL 100 and Kemira Superfloc A120 polymer as coagulation chemicals with chemical pumps producing -0.01 1/min and -0.004 1/min flows. The load of these chemicals are 1.8 g/1 and 0.4 mg/1, respectively. Calcium hydroxide milk injected with a similar pump to increase pH to 11+ and LD is injected with a robust peristatic pump so its concentration in the flow is 1 g/1 (flow is 0.07 1/min) .

The raw wastewater flow has -1000 mg-SS/1 and the settler overflow has -200 mg-SS/1.

The method and apparatus are suitable in dif- ferent embodiments for using in different solid separation processes. The method and apparatus are suitable in different embodiments for separating effectively solids from different liquid waste streams.

The invention is not limited merely to the em- bodiments and examples referred to above; instead many variations are possible within the scope of the inventive idea defined by the claims.