applied sciences Article A Preliminary Study on the Use of Xylit as Filter Material for Domestic Wastewater Treatment Marcin Spychała *, Tadeusz Nawrot † and Radosław Matz Department of Hydraulic and Sanitary Engineering, Poznan University of Life Sciences, Pi ˛atkowska94A, 60-649 Pozna´n,Poland; [email protected] (T.N.); [email protected] (R.M.) * Correspondence: [email protected] † Co-author Tadeusz Nawrot passed away before publication. Featured Application: The study indicated xylit as a material highly effective in wastewater qual- ity indicators removal. The material is suitable for simple construction (low cost) on-site wastew- ater trickling filters. Abstract: The aim of the study was to verify two morphological forms (“angel hair” and “scraps”) of xylit as a trickling filter material. The study was carried out on two types of polluted media: septic tank effluent (STE) and seminatural greywater (GW). The basic wastewater quality indicators, namely, chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total suspended solids (TSS), ammonium nitrogen (NNH4), and total phosphorus (Ptot) were used as the indicators of treatment efficiency. Filtering columns filled with the investigated material acted as conventional 3 3 trickling filters at a hydraulic load of 376–472 cm /d during the preliminary stage, 198–245 cm /d during stage I, and 184–223 cm3/d during stage II. The removal efficiency of the two morphological forms of xylit did not differ significantly. The average efficiencies of treatment were as follows: for Citation: Spychała, M.; Nawrot, T.; Matz, R. A Preliminary Study on the COD, over 70, 80, and 85% for preliminary stage, stage I and stage II, respectively; for BOD5, 77–79% Use of Xylit as Filter Material for (preliminary stage); for TSS, 42% and 70% during the preliminary stage, and 88, 91, and 65% during Domestic Wastewater Treatment. stage I; for NNH4, 97–99% for stage I and 36–49% for stage II; for Ptot, 51–54% for stage I and 52–56% Appl. Sci. 2021, 11, 5281. https:// for stage II. The study demonstrated that xylit was a material highly effective in wastewater quality doi.org/10.3390/app11115281 indicators removal, even during the initial period of its use. Academic Editors: Ioanna Vasiliadou, Keywords: greywater; nutrient; organic compound; wastewater; xylit Noori MD Tabish and Athanasia Tekerlekopoulou Received: 29 April 2021 1. Introduction Accepted: 31 May 2021 Published: 7 June 2021 Individual wastewater systems are common in many regions of Europe and some parts of the world (USA), due to lack of economic justification or sewerage systems; its Publisher’s Note: MDPI stays neutral building needs will develop. with regard to jurisdictional claims in Septic tanks are in general devices used for preliminary wastewater treatment; how- published maps and institutional affil- ever, they are prone to periodically increased concentrations of suspended solids in effluent, iations. which may accelerate clogging processes. Clogging, it should be noted, can occur even after a few years [1]. In view of the disadvantages of septic tanks and aiming to prevent outflow from small wastewater treatment facilities from instability and exceeding recommended or required limits, there is a need to look for systems of better treatment, including nutrients, which Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. can ensure longer life of existing soil infiltration systems. Some interesting and promising This article is an open access article solutions are referred to in the literature. They utilise, e.g., a geotextile fabric filter [2], a distributed under the terms and modified up-flow septic tank, followed by an anaerobic baffled reactor [3] and a gradual conditions of the Creative Commons chamber using a gravel bed as an effluent filter [4]. Attribution (CC BY) license (https:// The authors are therefore of the opinion that there is a need to look for trickling filter creativecommons.org/licenses/by/ filling materials that are cheap, highly effective in removing contaminants, and where 4.0/). possible, ones that have a low (negative) impact on the environment (carbon footprint). Appl. Sci. 2021, 11, 5281. https://doi.org/10.3390/app11115281 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5281 2 of 17 Such systems should be as cheap and simple as possible, especially in less developed or developing countries. The traditional trickling filter technology preceded by a septic tank or preliminary settler still seems to be a promising solution because of its simplicity and resistance to load and hydraulic overloading. In contrast, plastic carriers being used for trickling filter filling are relatively expensive and are often associated with negative side effects on the environment during their production, such as energy consumption and carbon footprint. Xylit (usually referred to as xyloid lignite) is a waste material (by-product) obtained from the mining of lignite (brown coal). Xylit is a type of lignite, sometimes referred to as fossil wood. The use of the term is justifiable because xylit frequently has an appearance similar to wood; some properties (and visual or physical features) of “wood” are altered [5]. The structure of carbonised wood fibres can be observed in this material of density around 400 kg/m3. It is not used as fuel for heat generation due to very low heat content (even in a dried state) and due to the obstruction of the technological process of construction ceramics production [6]. Xylit is not well recognised as a filtering material for wastewater or greywater treat- ment and only a few studies have been carried out on this usage [5,7]; however, it is used as a biocarrier in some decentralised wastewater systems [8]. One of the most detailed research was conducted by Zhang [9]. This author highlighted several interesting and important features of xylit that can prove useful in polluted water treatment, such as (1) par- ticle size of 20 mm, (2) solvent-accessible area (surface area approx. 2.5 m2/g), (3) pore volume of 0.01 cm3/g, (4) average pore size of about 16–17 nm, (5) overall negatively charged surfaces and molecular weight, and (6) positively correlated removal of micro- pollutants by xylit (with both apolar and polar surface area ratio, and pH value favouring bacteria growth). These all can be considered as features also useful for dissolved organic compound removal. For the purposes of the research experiment, xylit was crushed and sieved to obtain the relevant particle size of 2–4 mm [10]. Xylit proved to be a highly effective DOC removal material, it removed 52% DOC during the screening experiment and 89% DOC in the long-term experiment [9]. In that study, only granular activated carbon was better in DOC removal efficiency (average removal efficiency above 95%). Xyloid lignite (xylit) was examined in a column experiment involving the removal of 31 selected organic micropollutants and phosphorus using several sorbents over a period of 12 weeks, and the average removal of this material was 80 ± 28% [7]. This material proved to be less effective in removing micropollutants, as compared to GAC in the long-term column experiment; however, it demonstrated a higher removal efficiency of MPS than lignite and sand [9]. Despite a smaller surface area, xylit revealed higher removal efficiency than lignite. Chemically, xylit consists mainly of carbon and oxygen, therefore is not efficient in phosphorus removal by precipitation, as shown by [9]; in his study, Ptot was poorly removed by xylit with an efficiency of 14–22%. The role of adsorption or biological processes (biofilm development) in phosphorus removal with the use of xylit was considered in the study by Zhang et al. [7]. Zhang et al. [11] observed that the hydrophobic effect played an important role in the removal of micropollutants. Vargas et al. [12] noted that the presence of surface functional groups and the surface charge affect the adsorption capacity and the micropollutant removal mechanism. Lifespan (life cycle) has not been described in the literature. The potential reuse or transformation can be through its processing to compost or its reuse after heating it to a temperature around 300 ◦C with the aim of immobilising the adsorbed organic compounds. Despite some disadvantages, such as difficulties in uniform liquid sprinkling, the advantage of xylit is the ease of its processing. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 16 Despite some disadvantages, such as difficulties in uniform liquid sprinkling, the advantage of xylit is the ease of its processing. 2. Materials and Methods 2.1. Laboratory Setup (Model) and Experiment Conditions The experimental set was located in the laboratory of the Department of Hydraulic and Sanitary Engineering, Poznań University of Life sciences. The study was carried out Appl. Sci. 2021, 11, 5281 3 of 17 at a temperature close to room temperature (17–27 °C) from 7 May 2019 until 10 July 2020. 2. Materials and Methods 2.1. Laboratory Setup (Model) and Experiment Conditions 2.2. DescriptionThe experimental of setthe was Model located inand the laboratory Its Operation of the Department of Hydraulic and Sanitary Engineering, Pozna´nUniversity of Life sciences. The study was carried out at a temperatureThe research close to room model temperature consisted (17–27 ◦C) from of 7filtration May 2019 until columns 10 July 2020. (tubes) which were 100 cm long and2.2. had Description an ofinternal the Model and diameter Its Operation of 4.4 cm, made of organic glass (Figure 1). The depth of fillingThe with research the model filtering consisted material of filtration columns was (tubes)80 cm. which were 100 cm long and had an internal diameter of 4.4 cm, made of organic glass (Figure1). The depth of filling with the filtering material was 80 cm. Figure 1. Experimental set-up: (1—transparent acrylic glass tube (L = 100 cm; Din = 4.4 cm); 2—bed Figureof xylit 1.