Assessment of the technological reliability of a hybrid constructed wetland for wastewater treatment in a mountain eco-tourist farm in Poland

Evaluation of the technological reliability of individual wastewater treatment systems should be an important part of planning and decision-making in water and wastewater management, particularly now, when a wide range of technological solutions are available (Jóźwiakowski et al. 2015). Reliable operation of wastewater treatment units must be ensured due to both environmental and human health protection concerns (Eisenberg et al. 2001; Wojciechowska et al. 2016). Reliable operation of domestic wastewater treatment plants (WWTPs) is especially important in mountain aquifer areas, which are very sensitive to pollution. Mountainous regions in Poland play a key role in developing and sustaining vital socio-economic and environmental functions, which intertwine in the activities of spa and tourist resorts, forest management, environment-friendly farming and especially in hydrology. Although Polish mountainous areas have a high water production potential, as manifested by an almost 40% regional surplus in relation to the water discharged by all the rivers, Poland occupies one of the last positions in Europe in terms of the total freshwater resources, which are estimated at 1,630 m³·year⁻¹·person⁻¹. Therefore, it is essential that groundwater and the upper reaches of local streams and mountain rivers should be protected against biological, chemical and bacteriological pollution resulting from the discharge of untreated domestic sewage to the environment in rural areas. One of the key issues in technological progress in wastewater management in rural, environmentally valuable areas is the construction of small domestic or local WWTPs with a high, long-term reliability and effectiveness in reducing wastewater pollutions (Jucherski & Walczowski 2012; Masi et al. 2013; Gajewska et al. 2015; Jóźwiakowski et al. 2016).

Nowadays, more and more innovative wastewater treatment systems are being designed and offered on the market, which spurs the need for developing a universal method of assessing the reliability of the various treatment processes and facilities. Such a method would be helpful in planning and comparing the levels of protection offered by the various technologies (Jóźwiakowski et al. 2015).

The methods for determining the reliability of individual wastewater treatment systems have been described in more detail by Eisenberg et al. (2001). Lately, Djeddou & Achour (2015) have proposed a method for predicting reliability using artificial neural networks.

A comprehensive and useful assessment of the reliability of domestic WWTPs should be based on a series of measurements and observations of treatment variability under normal and critical operating conditions as well as the probability of mechanical failures and their impact on the quality of the treated wastewater (Eisenberg et al. 2001).

Reliability is defined as the probability of achieving required performance of a WWTP over a specific time and under specific conditions (Oliveira & Sterling 2008). To assess WWTP, a coefficient of reliability which relates mean pollutant concentrations to effluent standards (Niku et al. 1982) can also be used.

The statistical step in the assessment of WWTP reliability presented in this article is based on the normal (Niku et al. 1982), the lognormal (Oliveira & Sterling 2008) and the Weibull distributions (Wałęga 2009; Bugajski et al. 2012; Nastawny & Jucherski 2013). Statistical distributions of probability are used to establish the probability of occurrence of selected values of pollutants. Recent findings reported in the literature (Wałęga 2009; Bugajski et al. 2012; Bugajski 2014; Nastawny & Jucherski 2013) show that the Weibull distribution is an accurate and precise tool for evaluation of WWTP reliability.

Systems for domestic wastewater management are individual facilities with technical and quasi-technical treatment devices designed to collect and treat wastewater to the extent required by specific regulations (Regulation of the Polish Minister of Environment 2014) as well as discharge it to receivers without adversely affecting the soil–water complex in the place where the facilities are located. In rural areas, individual treatment facilities usually operate without constant supervision and are therefore particularly exposed to fluctuations in efficiency due to a variable load pattern (Platzer & Mauch 1997; Massoud et al. 2009); in consequence, the quality of effluent often does not meet the requirements stipulated in the regulations in force.

Hybrid wetland systems have recently been more and more frequently used for the treatment of domestic wastewater from museum buildings and forester or mountain shelters, including those located in national parks. When well designed and properly maintained, they can achieve high pollutant removal efficiencies (Masi et al. 2007; Osaliya et al. 2011; Jóźwiakowski et al. 2014, 2016; Sanchez-Ramos et al. 2015; Gizińska-Górna et al. 2016). The literature, however, provides little data on the reliability of pollutant removal processes in such constructed wetland systems (CWS) over long periods (years) of operation.

The aim of the present study was to assess the range of long-term fluctuations in treatment reliability in a domestic hybrid WWTP. The plant had been built over 10 years before on an eco-tourist farm in the mountain municipality and spa of Krynica-Zdrój in Poland and had been continuously operating since that time. The following indicators of wastewater contamination were measured: (i) biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD) for contamination with organic matter, (ii) total suspended solids (TSS), (iii) total nitrogen (TN), and (iv) phosphorus PO₄-P. The indicators' results (pollutant concentrations) were compared with the Polish standards for treated wastewater (Regulation of the Polish Minister of Environment 2014) discharged into water and soil from treatment plants below 2000 PE.

The installation consisted of a classic three-chamber septic tank, a vertical flow (VF) reactor (filter) followed by a special constructed wetland, and a permeable water pond as a receiver of the final effluent. The septic tank, with a capacity of 6 m³, was a monolithic concrete structure designed to receive sewage with a seasonal variability from 3 to 15 persons (with an average of PE = 5). The substrate used in the VF reactor were granules of a sintered clay material (KERAMZYT). To improve biological treatment and nitrification at the vertical stage, the wastewater was sprayed onto the surface of the reactor. Due to the harsh climate, it was necessary to use a cover to insulate the reactor from freezing in winter. Downstream of the reactor, there was a special filter bed (wetland) with subsurface flow, planted with a mix of reed sweet grass (Glyceria maxima), reed canary grass (Phalaris arundinacea) and other grasses spontaneously inhabiting the bed. The bed was constructed on a slope for tertiary treatment and removal of the remaining nutrients (N and P). The installation ended with a permeable pond for receiving and infiltration of the treated wastewater into the soil complex surrounding the treatment unit.

The average hydraulic load of the treatment plant in the investigation period was 621.2 dm³ day⁻¹, and the average concentrations of pollutants in raw sewage were 271.7 mg O₂·dm⁻³ for BOD₅, 390.8 mg O₂·dm⁻³ for COD, 75.4 mg·dm⁻³ for total suspended solids, 110.5 mg·dm⁻³ for TN, and 12.3 mg·dm⁻³ for phosphorus PO₄-P (Table 1). The flow of treated wastewater was calculated based on the water consumption reading of a water meter. BOD₅ was determined using the OxiTop respirometric measuring system from WTW. COD, TN and phosphate phosphorus were determined using a Merck SQ118 photometer and a Merck thermoreactor TR-200. The total suspended solids were determined by a gravimetric method, in accordance with the standard PN-72/C-04559/02.

Reliability was determined from cumulative distribution plots, taking into consideration pollutant concentrations in treated wastewater permitted by the Regulation of the Polish Minister of Environment (2014), i.e.: BOD₅ ≤ 40 mg O₂·dm⁻³, COD ≤ 150 mg O₂·dm⁻³, total suspended solids ≤50 mg·dm⁻³, TN ≤ 30 mg·dm⁻³, total phosphorus ≤5 mg·dm⁻³.

The many-year means of effluent contaminant concentrations in wastewater treated in the investigated installation (BOD₅ – 3.49 mgO₂·dm⁻³, COD – 28.0 mgO₂·dm⁻³, suspended solids – 15.8 mg·dm⁻³, TN – 20.9 mg·dm⁻³, and phosphorus P-PO₄ – 1.70 mg·dm⁻³) (Table 1) were much lower than required by the Regulation (2014) on discharging wastewater into the soil or surface waters including lakes and their tributaries, and artificial water reservoirs situated on flowing waters.

Table 1 shows the basic statistics for pollutant concentration in wastewater.

Statistical analysis using the Kolmogorov–Smirnov goodness-of-fit test showed that the Weibull and lognormal distributions could be well fitted to the empirical distributions of each pollution indicator, whereas the normal distribution gave a much worse fit, although it could also be used for most of the parameters, except BOD₅ (Table 2).

The technological reliabilities of the installation determined by the Weibull distribution function compared with the lognormal distribution are given in Table 4. It was shown that the reliability of reducing BOD₅ and COD concentrations was 100% for both the lognormal as well as the Weibull distribution functions. For other pollutants, the reliabilities of the treatment plant described by the lognormal distribution and the Weibull distribution were slightly lower at 92.6% and 99.5% for TSS, 77.0% and 76.8% for N_tot, and 95.2% and 98.2% for phosphorus PO₄-P, respectively (Figure 3, Table 4).

Compared with the technological reliabilities of domestic WWTPs evaluated by other authors (Wałęga et al. 2008; Bugajski et al. 2012), the installation investigated in this study was much more efficient and reliable as far as the removal of pollutants was concerned. The reliability of reducing the levels of organic matter in wastewater was 100% for BOD₅ and COD and more than 92% for suspended solids. The corresponding values for the treatment plant Biocompact BCT S-12 (with activated sludge) were 68% (BOD₅), 88% (COD) and 62% (suspended solids) (Bugajski et al. 2012). In the case of the domestic treatment plant RetroFAST (with an aerated biological filter), the reliability values were 85%, 89% and 92%, respectively (Wałęga et al. 2008). Wastewater from individual rural households collected in septic tanks is characterized by several times higher concentrations of pollutants than wastewater discharged by municipal sewage systems. The results of a two-year monitoring study of the quality of wastewater, conducted in one of Polish villages, are shown in Table 6.

Due to this fact, the technological set up of a single-family WWTP (below 50 PE) needs to be characterized by a very high pollutant removal efficiency as well as a very high resistance to these fluctuations. Such requirements are practically impossible to meet using container WWTPs with activated sludge or trickling filters. By contrast, hybrid systems equipped with beds built as treatment wetlands can easily adapt to such fluctuations and ensure stable removal of pollutants from wastewater.

As far as BOD₅ and COD are concerned, the analyzed installation worked without technological failures over the whole 10-year study period. For suspended solids, the probability of occurrence of failure events (effluent quality parameters higher than permitted) was two days per year and for phosphorus seven days per year. The technological reliability of TN removal was much lower (76.8%), with as many as 85 failure days during the whole year. The hybrid treatment plant had operated continuously for more than 10 years and had been maintained to ensure constant operational availability. The yearly removal efficiency of pollutants was sensitive only to a slight periodic variability of both hydraulic and pollution loads caused by tourists staying at the farm during vacation periods. The plant had never been observed to freeze in winter thanks to the snow cover, and the only decrease in average efficiency in the cold season concerned the removal of N_tot (18%) (Jucherski & Walczowski 2012). These malfunctions occurred in winter, when the weather conditions were not conducive to efficient denitrification of NO₃-N, which was the dominant form of nitrogen in the effluent (Figure 4). By contrast, the installation showed a very high rate of conversion of ammonium nitrogen. The average concentration of NH₄-N did not exceed 2.1 mg·dm⁻³ over the entire research period. In order to increase the efficiency of tertiary wastewater treatment in winter seasons and thereby improve the overall pollutant removal efficiency of the WWTP, further efforts have to be made at re-designing and re-building the existing filter bed.

To summarize, the installation tested can be particularly recommended for use in mountainous regions, where streams and rivers as well as underground waters are very sensitive to pollution and, therefore, require higher levels of protection. The high reliability of this type of WWTPs is a consequence of the application of a hybrid configuration of facilities, which is characterized by an increased technological inertia in the multi-staged wastewater treatment process (Jucherski 2007). The number and configuration of the facilities constituting the wastewater treatment installation have been chosen so as to ensure the stability of the process under changeable weather conditions and variable pollutant loads in raw wastewater. The advantages of the investigated installation include simple operation, low power consumption and low operating costs. One disadvantage is that, compared with a container WWTP, the system occupies a slightly larger surface area, which, however, does not limit its application, especially in rural (Nastawny & Jucherski 2013) or protected areas (Jóźwiakowski et al. 2014; Jóźwiakowski et al. 2016). The design and structure of the test facility make it especially suitable for use in sloping terrain with large inclines typical of mountainous regions.

This study showed that the technological solutions applied in the investigated installation for the treatment of wastewater produced by an eco-tourist mountain farm proved to be very effective and reliable during the whole 10-year period of operation. The long-term median concentration values of effluent pollutants (BOD₅ – 2.5 mg O₂·dm⁻³, COD – 25.0 mg O₂·dm⁻³_, N_tot – 19.5 mg·dm⁻³, PO₄-P – 1.5 mg·dm⁻³ and suspended solids – 14.0 mg·dm⁻³) were lower than permitted by the Polish Regulation (Regulation of the Minister of Environment 2014). At the same time, the specific loads of pollutants in the effluent were much lower than those specified in the HELCOM Recommendation.

The technological reliability of the tested installation (100% for both BOD₅ and COD removal, over 90% for the removal of PO₄-P and total suspended solids, and 77% for TN removal), calculated with the Weibull method, confirmed that the treatment plant could be used as an effective tool for protecting the quality of local water resources (especially in ecologically valuable mountainous areas) regardless of changeable weather conditions and variable loads of pollutants characteristic of individual wastewater management in rural regions.

The statistical methods based on the Weibull as well as the lognormal data distributions describe very well the degree of stability and technological reliability of treatment processes, and the differences between them in estimating reliability are negligible.

The Weibull method is especially well-suited for comparing the specific functional features of various types of rural domestic WWTPs, but the log-normal distribution can also be used.