Using a compact unit for nitrification and denitrification processes followed by activated carbon filtration to remove total nitrogen from ammonium-contaminated groundwater

Groundwater extraction is one of the utmost important techniques of water supply, considering that groundwater is preferred to surface water for water supplies since it is pure and consistent in quality and quantity year-round. About 50% of the world's drinking water supply comes from groundwater (Smith et al. 2016). Additionally, groundwater is used for agricultural (Kamal El-Din et al. 2021; Zhao et al. 2021) and industrial purposes (Amiri et al. 2021; Maharjan et al. 2021, 2022; Rao et al. 2021). However, anthropogenic groundwater contamination and recharging through leakage into the water supply network, municipal sewage, and on-site waste disposal are causing significant problems in both developed and developing countries (Lerner 1990; Wakida & Lerner 2005; An et al. 2016; Schwarz & Mathijs 2017; Ferronato & Torretta 2019; Bulut et al. 2020; Li & Tabassum 2021). These concerns are worse in locations where groundwater is the main source of freshwater (Umezawa et al. 2008; Gao et al. 2012) and in the densely populated communities of developing countries due to inadequate infrastructure (Nyenje et al. 2014; Grimmeisen et al. 2016). Different organic and inorganic substances easily impact the groundwater environment (Vidal et al. 2000; Ahmed et al. 2011; Shakya et al. 2019). As a result, groundwater quality control is crucial to ensuring that the hygienic requirements and standard specifications for drinkable water are met. In situ treatment may occasionally need to be specified in the extracted groundwater (Yevate & Mane 2017; El-Sayed 2018; Kamal El-Din et al. 2021).

Groundwater contamination from ammonium-nitrogen (⁠⁠) poses serious environmental and public health problems (Patterson et al. 2002; Huang et al. 2015; Maharjan et al. 2020, 2021). Groundwater naturally contains due to the anaerobic degradation of organic matter and artificial due to the disposal of organic waste (Böhlke et al. 2006). Groundwater can sometimes have undesirable qualities, such as an offensive odor and bad taste, when its concentrations exceed the World Health Organization's acceptable limit for potable water, which is 1.2 mg/L. As a result, the groundwater is no longer suitable for drinking (World Health Organization 2017; Maharjan et al. 2022). Furthermore, may cause fast biological regrowth in distribution systems and produce cancerous nitrogenous disinfection byproducts while disinfecting potable water (Khanitchaidecha et al. 2012; Zhang et al. 2014; Huang et al. 2015; Maharjan et al. 2021). Therefore, eliminating from groundwater is crucial for controlling water contamination and health risks (Huang et al. 2015; Maharjan et al. 2021). On the other hand, the nitrification also leads to the conversion of to generated nitrite (⁠⁠) and finally nitrate (⁠⁠) (Soares 2000; Khanitchaidecha et al. 2012; Aloni & Brenner 2017; Maharjan et al. 2022). Due to methemoglobinemia or ‘blue baby syndrome,’ nitrate (⁠⁠) is regarded as a dangerous substance, while nitrite (⁠⁠) can also result in a carcinogenic compound. Therefore, total nitrogen (TN) removal (and not just removal) from ammonium-contaminated groundwater before the conventional water treatment procedure is very important (Soares 2000; Khanitchaidecha et al. 2012; Aloni & Brenner 2017; Maharjan et al. 2022).

In order to remove from groundwater, various techniques are applied. Examples include biological techniques like biofilters, trickling sand filters, permeable reactive barriers, and dropping nitrification (Štembal et al. 2005; Gibert et al. 2008; Robertson et al. 2008; De Vet et al. 2009; Maharjan et al. 2020, 2022) as well as physicochemical techniques such as ion exchange, reverse osmosis, and adsorption (Wang et al. 2007; Šiljeg et al. 2010; Kubal et al. 2011; Maharjan et al. 2022). In general, biological techniques are more economical and environmentally friendly than physicochemical techniques (Huang et al. 2018; Maharjan et al. 2020, 2022). Dropping nitrification is one of the most effective, affordable, and sustainable methods for removing from contaminated groundwater, as reported by Maharjan et al. (2020) and Maharjan et al. (2022).

There are numerous ways such as physical, chemical, and biological that can be used to eliminate generated by nitrification. Biological removal (denitrification) is considered the most cost-effective and environmentally safe and feasible on a large scale. Denitrification is the reduction of to nitrogen gas (N₂) carried out by aerobic bacteria, which, in the absence of dissolved oxygen (DO), can use as a terminal electron acceptor (Rezvani et al. 2019; Li & Tabassum 2021; Benrachedi et al. 2022; Maharjan et al. 2022). Denitrification generally occurs in an anoxic or anaerobic environment containing sufficient dissolved organic carbon. However, natural groundwater contains almost no organic carbon due to the filtration of soil. Therefore, the addition of an organic carbon source to water has become the main technical problem of in situ biological nitrogen removal (heterotrophic denitrification). Glucose, methanol, ethanol, and acetic acid are examples of liquid carbon sources that are frequently employed as conventional carbon sources. However, adding a liquid carbon source is expensive, and there is a chance of overdosing on potentially dangerous substances. Additionally, it is more challenging to regulate the carbon source dosage, particularly when the influent nitrate changes (Soares 2000; Xu et al. 2009; Li & Tabassum 2021).

An alternative source of electron donors for denitrification is solid organic carbon. Cellulose is most likely the solid carbon source to become a new carbon source. Whereas to promote biological denitrification, an organic carbon supply is necessary (Trudell et al. 1986; Aloni & Brenner 2017; Li & Tabassum 2021). Utilizing a solid carbon source eliminates the need to pump a liquid organic solution into the bioreactor, which reduces the concern of residual organics leaking into the process effluent. Furthermore, the consistent distribution of organic matter prevents the total organic carbon (TOC) gradient along the denitrification biofilters (Boley et al. 2000; Ashok & Hait 2015; Aloni & Brenner 2017). Numerous sources of solid carbon have been investigated, such as cotton wool, newspapers, and starch polymers (Volokita et al. 1996; Singer et al. 2008; Aloni & Brenner 2017; Benrachedi et al. 2022; Maharjan et al. 2022). Since cotton wool is widely used in agriculture and is inexpensive, it is regarded as a sustainable and effective carbon source for heterotrophic denitrification (Aloni & Brenner 2017; Maharjan et al. 2022). However, the outlet samples of the denitrification units using cotton media are unfit for drinking due to the increased TOC in them. Hence, more research is needed to determine how to use post-treatment techniques such as activated carbons and sand filters to decrease the release of TOC in the treated water.

The innovative method of dripping nitrification–cotton-based denitrification reactor is efficient, sustainable, low cost, reduced energy consumption, and a promising option for removing TN from -contaminated groundwater (Maharjan et al. 2020, 2022). Maharjan et al. (2022) demonstrated that the total N removal rate was 58.1–66.9 mg-N/d, while the efficiency of the cotton-based dropping nitrification reactor was 96–98%, which can be achieved by operating the dropping nitrification–cotton-based denitrification reactor for 91 days.

The present study aims to determine the ability of a compact unit for nitrification and denitrification processes, which consists of a dripping nitrification reactor followed by a denitrification biofilter to remove plus generated ⁠, and finally from groundwater. Furthermore, activated carbon filtration was applied as a post-treatment approach to remove the released TOC after denitrification biofilter using cotton media.

The industrial zone of Quesna, Menoufia Governorate, is located approximately 60 km north of Cairo, Egypt. The fourth phase of this industrial zone depends on feeding water from underground wells, where raw groundwater entering the water treatment plant is supplied to produce 2,400 m³/day of potable water. In this study, raw groundwater was used in experiments for nitrification and denitrification processes. Samples of raw groundwater were collected, and the measurements due to the experimental program were completed within 12 months. Table 1 represents the raw groundwater characteristics and specifications of potable water according to World Health Organization (2017).

Table 1

The raw groundwater characteristics and the standard values of potable water

Parameter .	Raw groundwater .	Standards of potable watera .
pH	7.68 ± 0.09	6.5–8.5
DO, mg/L	2.37 ± 0.18	–
Turbidity, NTU	0.8 ± 0.86	1.0
TDS, mg/L	134.8 ± 50.4	500
Calcium (Ca2+), mg/L	19.5 ± 8.4	≤75
Magnesium (Mg2+), mg/L	9.06 ± 6.3	≤30
Chlorides (Cl−), mg/L	45.8 ± 13.2	≤250
Sulfates (⁠⁠), mg/L	9.1 ± 5.4	≤200
Ammonium-nitrogen (⁠⁠), mg/L	29.8 ± 1.9	≤0.5
Nitrates (⁠⁠), mg/L	Nil	≤45
Nitrite (⁠⁠), mg/L	Nil	≤0.2
TOC, mg/L	Nil	≤2.0

Parameter .	Raw groundwater .	Standards of potable watera .
pH	7.68 ± 0.09	6.5–8.5
DO, mg/L	2.37 ± 0.18	–
Turbidity, NTU	0.8 ± 0.86	1.0
TDS, mg/L	134.8 ± 50.4	500
Calcium (Ca2+), mg/L	19.5 ± 8.4	≤75
Magnesium (Mg2+), mg/L	9.06 ± 6.3	≤30
Chlorides (Cl−), mg/L	45.8 ± 13.2	≤250
Sulfates (⁠⁠), mg/L	9.1 ± 5.4	≤200
Ammonium-nitrogen (⁠⁠), mg/L	29.8 ± 1.9	≤0.5
Nitrates (⁠⁠), mg/L	Nil	≤45
Nitrite (⁠⁠), mg/L	Nil	≤0.2
TOC, mg/L	Nil	≤2.0

^aAdapted from National Research Council (2007) and World Health Organization (2017).

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The dripping feed unit of raw groundwater with a flow rate of 5 L/day supplied the nitrification reactor, resulting in a hydraulic rate of 0.18 mL/min in the dripping nitrification reactor, noting that this flow rate is within the limits from 4 to 9 L/day which was recommended by Khanitchaidecha et al. (2012). After completing nitrification, the denitrification biofilter received the effluent water from the nitrification reactor. The denitrification biofilter was made of two Plexiglas columns each measuring 50 cm in height and 10 cm in internal diameter, and they were filled with 100 g of treated cotton as illustrated in Figure 1. The treated cotton fiber (from Misr Spinning and Weaving Company, El-Mahalla El Kobra. Egypt) passed through several processes to produce a high-quality absorbent to be utilized as a denitrifying bacteria holder. Furthermore, the compact unit for nitrification and denitrification processes was followed by an activated carbon filter from AQUA^® with cartridge dimensions of 2.5 cm in diameter and 50 cm in height as illustrated in Figure 1. The activated carbon media have a volume of 0.5 cm³/g and a specific surface area of 900 m²/g.

The dripping nitrification reactor and the denitrification biofilter were incubated with 10 L each separately of activated sludge from the adjacent wastewater treatment plant for 7 days to colonize each of the nitrifying bacteria in the dripping nitrification reactor and the denitrifying bacteria in the denitrification biofilter before filling it. Samples were taken from the inlet, outlet, and sampling points of the compact unit for nitrification and denitrification as well as the activated carbon filter as shown in Figure 1. The measurements included pH, DO, oxidation–reduction potential (ORP), ⁠, ⁠, ⁠, and TOC, which all were measured every 10 days through 120 days of the experimental program.

The pH and total dissolved solids (TDS) were measured using the Combo pH/EC/TDS/Temp tester, model HI98129 – Hanna Instrument, Egypt. The DO and ORP were measured using the oxygen meter model 8401 and ORP meter model 8551, respectively, from AZ Instrument Corp., Taiwan. Furthermore, the turbidity was measured using the Hanna portable turbidity meter model HI-83749-02 – Hanna Instrument, Egypt. The TOC concentration was measured using the TOC-LCSH/CSN Shimadzu TOC analyzer, Shimadzu, Japan. The rest of the analyses were performed using a device of PACK TEST from Kyoritsu Chemical-Check Lab., Corp., Japan. was measured using the indophenol method, while the naphthyl ethylenediamine method was applied for and measurements. All analyses were completed according to the Standard Methods for the Examination of Water and Wastewater, 23rd Edition (APHA 2017). Each experiment in this study included at least three replications to decrease the probable errors as much as possible.

Referring to Figure 4 and Figure 7, it is noticed that the in raw groundwater has been converted to and through the dripping nitrification reactor, and then ⁠, and have been disposed of, releasing nitrogen gas in the denitrification biofilter.

The scope of the present study intends to determine the ability of a compact unit for nitrification and denitrification processes, which consists of a dripping nitrification reactor followed by a denitrification biofilter to remove plus generated ⁠, and finally from groundwater. Furthermore, activated carbon filtration was applied as a post-treatment approach to remove the released TOC after the denitrification process. The following notable conclusions were attained:

• The outlet DO and ORP values increase significantly more than the inlet DO, and ORP values in the dripping nitrification reactor. Increased DO and ORP levels endorsed microbial nitrification by promoting an aerobic environment. On the other hand, the outlet DO and ORP levels decreased substantially more than the inlet DO and ORP values in the denitrification biofilter.

• It has been found that the outlet pH values decrease significantly more than the inlet pH values in both the dripping nitrification reactor and the denitrification biofilter. This occurs as a result of nitrifying bacteria eliminating in the nitrification process. However, pH reduction in the denitrification biofilter occurs as a result of proton generation during heterotrophic denitrification.

• The in raw groundwater has been converted to and through the dripping nitrification reactor, and then the and have been disposed of, releasing nitrogen gas in the denitrification biofilter. The nitrification efficiency reached 98.81%, while the denitrification efficiency reached 95.98%.

• The removal rate of TN in the compact unit for nitrification and denitrification processes ranged between 122.5 and 147 mg/d. Therefore, relatively higher discharges of groundwater can be accommodated by the compact unit for nitrification and denitrification processes, while maintaining the same efficiency in removing nitrogen in all its forms.

• The denitrification process utilizing cotton media resulted in a considerable increase in the outlet TOC concentrations, which ranged from 52.1 to 74.2 mg/L. In order to remove the released TOC from the denitrification process, post-treatment using an activated carbon filter was performed. The TOC concentrations in the filtered water from the activated carbon filter fully vanished after around 10 days of running.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.