Reuse of municipal wastewater for different purposes based on a modular treatment concept

Due to water scarcity and water pollution, the importance of water reuse is increasing more and more. AspartofaGermanresearchprogrammeonwaterreuse,theef ﬂ uentofawastewatertreatmentplantin the coastal region of northern Germany was used to investigate the direct treatment of tertiary ef ﬂ uent within the project MULTI-ReUse. A modular constructed pilot system has been operated to optimize different treatment chains producing different water qualities simultaneously. The technological focus was put on membrane technologies, namely ultra ﬁ ltration (UF) and reverse osmosis (RO), and also bio ﬁ ltration, adsorptionand disinfectionwere partofthe piloting. Besidethe developmentofmonitoring strategies forensuring biologicaland chemical safe water qualities,the operationalstabilityand the safe transportofwatertotheconsumerswereexamined.Thedirecttreatmentofwastewaterisademanding taskduetothelackofdilutionandhydraulicretentiontimeinthereceivingwater (environmentalbuffer). However, the multiple barrier approach guaranteed constant secure water. Fine adjustments of individual processes were particularly important. A stable operation of the UF could be realized in particular by using more or less intermitting inline coagulation as coating. The RO performance could be improved signi ﬁ cantly by using monochloramine as disinfectant to minimize biofouling. Implementation of an ultra-low-pressure reverse osmosis membrane (ULPRO).


INTRODUCTION
Water reuse can be of interest for various reasons. On the one hand, more frequent and severe drought conditions due to the climate change (Solomon et al. ), excessive water consumption and competing environmental, industrial and agricultural needs for water can stress the availability of conventional freshwater resources (Drewes & Horstmeyer ). On the other hand, the low quality of regionally available fresh water resources gives reason to focus on the use of alternative resources. In this context, collected wastewater has been explored as an additional water resource since decades (Asano et al. ). Up to date, numerous wastewater reclamation facilities have been successfully implemented for different water reuse purposes (Curl et al. ). Even the direct potable reuse is becoming more and more accepted as state-of-the-art technologies have proved to provide the desired water quality with high reliability.
Despite the successes that water reclamation projects have achieved internationally, in Germany this topic is still being discussed very critically and defensive by many public actors (UBA ). However, according to climate change predictions, more warm and dry summers are to be expected in central Europe. The effect on the groundwater level and surface water reservoirs could be noticed in several areas in Germany during the years 2018 and 2019 (Hellwig et al. ). The expected simultaneous rise in sea levels also leads to a disturbance in the balance between groundwater and the seawater interface, which results in increasing salinization of the groundwater body.
This makes the North Sea coast of Europe a good example for the problem of saltwater intrusion (CLIWAT, 2011).
On this background, a regional water board in Lower Saxony, Germany, decided to explore the possibilities of municipal wastewater reuse for industrial purposes in a local case study. Nordenham is a mid-sized German town with water-intensive industries and trades. It is located in the Wesermarsch, a coastal region in the northwest of Germany, which is affected by saltwater intrusion. The entire district does not have its own drinking water supply.
In the surrounding districts, drinking water obtained from groundwater resources is treated and pumped over long distances to the industrialized area. Here, the water demand for industrial processes with very low water quality requirements is covered as much as possible with water from the high salinated river Weser. However, for the majority of applications, drinking water is used (Kroemer et al. ).
To lower or to completely avoid the demand for drinking water in cooperating industrial companies could lead to a significant relief of the used drinking water aquifers. This case study was a main part of the MULTI-ReUse project of a German multicentre research consortium. Within the project, a treatment system for water reuse has been tested at the site of the municipal wastewater treatment plant (WWTP) of Nordenham with the aim of producing process water in three different qualities.
The technological studies focused on the membrane processes ultrafiltration (UF) and reverse osmosis (RO) supplemented by further treatment steps. Depending on the intended use, the processes can be combined into different treatment chains (Fit-for-Purpose) (EPA//R-/ ).
By choosing these state-of-the-art technologies were focused on the demonstration of technologies under site-specific conditions. Additionally, the development of new RO membranes and microbial monitoring methods was supported.
This article describes the set-up and the main results of the pilot plant operation, beside the aspects of waste disposal and water transport are discussed for the specific case study.

Pilot plant
The MULTI-ReUse pilot plant consists of modular units for three treatment lines ( Figure 1): (I) Line 1 for ReUse Water 1: pre-filtration (sieve), optional adsorption with powdered activated carbon (PAC), flocculation, UF and UV disinfection; water for rinsing and cooling purposes.
(III) Line 3 for ReUse Water 3: pre-filtration, optional PAC and flocculation, UF, chemical disinfection, RO and UV; water for industrial purposes.
Two UF lines were operated in parallel. The treatment lines were protected by a sieve filter (AMIAD TAF-750) with a mesh size of 200 μm. The sieve filter was rinsed automatically. Coagulation was used prior to UF to reduce fouling by decreasing the cake resistance, limiting pore blockage and increasing the backwash efficiency (Barbot et al. ). Coagulation also increases the removal of organic matter, phosphate and disinfection by-product After pre-treatment, the water was filtrated with two large-scale UF modules (dizzer XL 0.9 MB 80 WT, INGE; polyethersulphone, capillary inner diameter 0.9 mm; pore diameter 0.02 μm and membrane surface area 80 m 2 ). The flux varied from 60 to 70 L/(m 2 ·h), and the filtration time was set to 45 min. Chemical enhanced backwash (CEB) with NaOH (pH ∼12.2) and H 2 SO 4 (pH ∼2.1) was done one up to two times per day. Cleaning-in-place (CIP) intervals were depending on the amount of fouling but generally occurred one up to two times per year with 300 mg Cl free /L (pH ∼12.2) by dosing NaClO and 4 g of oxalic acid/L (pH ∼1.8). For ReUse Water 1, the filtrate was finally disinfected by UV radiation (sterilAir ® AQD64-4 K).
For ReUse Water 2, the water was further treated successively by inline aeration, biologically active deep bed filtration (bed height 2 m in two serial columns; quartz sand, grain diameter 1-2 mm; filtration rate 5.5 m/h, empty bed contact time (EBCT) 20 min) and GAC filtration To prevent massive biofouling on RO membranes and their pre-filters, NaClO and NH 4 Cl solutions were added consecutively on line 1 directly into the UF filtrate stream in the stoichiometric ration of 2:1. Both react in situ to 1 mg/L of monochloramine: The hydraulic retention time of monochloramine was, due to the 1.8 m 3 UF filtrate tank, approximately 23 min.
Free chlorine concentrations up to 0.1 mg/L were tolerated according to the RO manufacturer's requirements. Therefore, the dosage of sodium bisulphite as a scavenger for

Analytical methods and calculations
The chemical analytics of the water samples was done by the accredited laboratories of EUROFINS, IWW and OOWV. The analysis of the microbiological parameters was carried out by the laboratory of the Public Health Department of Aurich. The applied methods are listed in

Operation conditions
The pilot study started in August 2017. Up to December 2018, each treatment step was commissioned and optimized individually. In the period from January to December 2019, all treatment steps were operated in combination and further optimized to demonstrate the stability of the processes and the water quality.
In the WWTP of Nordenham, physical, chemical and biological processes are combined for the treatment of the municipal wastewater, which is collected from households, small and medium enterprises, and a hospital. The last treatment step of the WWTP is a clarifier. The sewer system consists of both separate and combined rainwater and sanitary sewers. In dry weather periods, the effluent volume flow is ∼4.000 m³/d. During rain events, it increases by the factor of three. The effluent shows the seasonal variations of high ammonium and nitrite peaks during periods with low temperature and can be characterized as a typical municipal WWTP effluent unaffected by big industry. One special characteristic of the raw and treated wastewater is the high content of dissolved manganese, which enters the sewer system with the groundwater. The results of the major physical-chemical parameters that have been monitored in a monthly routine are listed in Table 2.
As hygienic safety of customers and operators is a prevalent goal in water reclamation, a regular monitoring of traditional microbial hygiene indicator bacteria was performed. Figure 2 shows the results of this monitoring for the WWTP effluent.
Although the purpose for water reuse in the case study at Nordenham was not irrigation but industrial use, the elimination of micropollutants by RO and GAC has been investigated at the pilot plant, because water reuse for agricultural or landscape purposes is of interest in some areas in Germany. Table 1 shows a selection of micropollutants that were found in the wastewater effluent of the WWTP of Nordenham.

Produced water qualities: hygienic aspects
During the entire pilot phase, the UF proved to be an effi- This means, by looking at the total cell counts in the nonsterile system, that the UF retention is likely to be underestimated. As microbiological monitoring was a main focus Produced water qualities: physical-chemical parameters Results on the chemical composition of the water generated and the WWTP effluent are shown in Table 2.
The ReUse Water 1 is free of particles and pathogens.
Flocculation before the UF reduced the DOC concentration on average to 10 mg/L (À19%) and the concentration of P total to 0.061 mg/L. The concentrations of the metals such as aluminium and iron contained in the wastewater effluent were also significantly reduced by 61 and 93%, whereas dissolved salts pass through the UF membrane.
Further treatment of ReUse Water 1 by filtering it through a biological active quartz sand filter and a GAC filter leads to ReUse Water 2 quality. ReUse Water 2 is additionally low in manganese (À68%), ammonium (À70%) and organic micropollutants after the GAC passage. Figure 3 shows the removal efficiency of the GAC filter for several micropollutants at increasing operating times,  Figure 3, the progress of the breakthrough of some micropollutants can be observed. As one extreme the benzotriazoles can be mentioned, which were removed nearly to 100% over the complete operation time.
On the other side, at the end of the pilot phase, a certain mass of already adsorbed sulfamethoxazole had been replaced by competing organics with a desorption effect: the concentration sulfamethoxazole was higher in the fil-     can be assumed that the membrane will not recover on its own, but will be needed to be subjected to intensive cleaning. Operating problems of the WWTP, which were confirmed after consultation with the operator, also showed high stress levels for the UF process. A deterioration in the quality of the WWTP effluent can disturb immensely.
In some cases, the increase of the coagulant dosage rate and also increase of the CEB frequency (2 CEBs/d) were able to stop the increase of the TMP. It was finally necessary to switch off the pilot plant and to clean the UF with NaOH and NaClO (pH 12.2 and 300 mg Cl free /L).

)
Biofouling and scale formation can reduce the performance of the RO. To ensure operational stability for the production of ReUse water 3, monochloramine (NH 2 Cl) was used in addition to antiscalant for RO1 to avoid biofouling. RO2 served as a reference line without monochloramine disinfection. The dosage of monochloramine was noticeable, both in the differential pressure of the cartridge filter and as well in the permeability of the filtration process. Especially in the summer months due to high water temperatures of above 20 C and the highly concentrated inlet water (lack of rain water in the mixed water sewer system), the biological activities were clearly visible on the cartridge filters. At times with peak water temperatures, the cartridge filters had to be replaced after 2 weeks at the latest. By using monochloramine, this period could be extended up to 2 months. Furthermore, the cleaning of the UF filtrate tank (storage for backwash water) in 2-week intervals was no longer necessary. Figure 7 shows the effect of 1 mg NH 2 Cl/L for a dosing time of 22 h/d on the cartridge filter upstream to RO1 in comparison to that of RO2 over a period of 4 weeks (see Figure 8) in the hot summer of 2019. The deposition on these cartridge filters was examined by preparing a distinct piece of each cartridge with aqua regia solution and analysing the extract by ICP-OES. The summarized mass of all analytes in relation to the initial sample weight was significantly lower at RO1 (9.7 g/kg) compared with RO2 (23 g/kg).
The chemical elements with the highest share among all elements identified by this method were iron (RO1: 2.8 g/ kg; RO2: 11 g/kg) and calcium (RO1: 3.5 g/kg; RO2: 6.2 g/kg). The fact that the Fe deposition on the RO2 cartridge was about four times higher than on the RO1 cartrige and the amount of phosphorous was also significantly higher (RO1: 0.16 g/kg; RO2: 1.0 g/kg) confirms the assumption that the biological activity was effectively controlled by the disinfectant dosage.  The efficiency of the filtration operation via the RO was monitored via the process data measured online. Figure 8 shows the normalized permeate flow for RO1 (with One option, which was still being examined at the time this publication was written, is a discharge of RO concentrate into the River Weser. This seems to be the easiest way of disposal; however, a discharge permit is required. The Weser River is a navigable waterway and the river section at the case study site belongs to the estuary (brackish water). The costs for the discharge permit, which is depending on the quality and volume, should be considered. Backwash water of UF is largely unaffected by dissolved water additives. The hydroxides of the coagulation agent and PAC, if use is required, are the only water treatment substances added into the treatment process prior to the UF.
The particulate matter, including metal-containing flocs (Fe 3þ or Al 3þ ) and PAC, will be eliminated during the conventional WWT process. In addition, PAC stabilizes the biological treatment of the WWTP and its sludge treatment (dewatering) (Menzel ). Its loading capacity can be widely exploited because on the one hand the adsorption in the PAC-UF process does not reach equilibrium conditions and on the other hand the PAC will be applied (counter-current) in two treatment steps with a high concentration level during the biological treatment and a lowered level during the UF step (DWA ).  Regarding the service conditions, it has to be considered that they may differ between drinking water and reuse water.
In contrast to a drinking water network, the conditions in a reuse water networkespecially for industrial purposescan be characterized by a constant, plannable consumption.
The materials and conditions of the respective operating networks of the consumers of reuse waters also have to be taken into account, as well as the intended purpose of usage. Due to the great variety between the consumers, this can only be done by individual assessments and will not be further discussed here.

CONCLUSION
In the MULTI-ReUse project, a modular treatment concept for the reuse of WWTP effluent was developed. To ensure consistent stable water quality, reliable, i.e. robust treatment technologies were used on the one hand, and extensive online monitoring in combination with laboratory analyses on the other.
The importance of considering the treatment efficiencies of the WWTP and the reclamation plant as one unit has to be emphasized. There is still a lack of data for the evaluation of the influence of operational problems in the WWTP (e.g. in drought or heavy rain periods) on the membrane processes in the reclamation plant. It is also suspected that certain chemical water constituents entering the WWTP occasionally and not have been removed in the WWTP can disrupt the in/out UF filtration process. The experiments showed that under normal circumstances at least 6 mg Fe 3þ /L should be used for a stable filtration process when using FeCl 3 and at least 2 mg Al 3þ /L when using Al 2-(OH) 5 Cl·2-3 H 2 O as a coagulant. A combined coagulant coating strategy was successfully used to stabilize the UF process.
For protecting the RO membrane against biofouling and keeping the performance, high in situ formation of monochloramine prior to the RO membrane was highly effective. Furthermore, it suppresses the regrowth of bacteria in the RO permeate by passing the membrane and stabilizing the water. Especially in hot summers and drought periods, the potential of biofouling formation is quite high.
Consequentially energy and maintenance costs for intensive cleanings of membrane pipes can be reduced and the treatment be more efficient. The evaluation of possible materials for a distribution network has shown that in most cases individual assessments are necessary regarding the operating conditions and the purpose of usage.