Small Wastewater Treatment Plants Research Articles

Review of the Seasonal Wastewater Challenges in Baltic Coastal Tourist Areas: Insights from the NURSECOAST-II Project

The NURSECOAST-II project addresses the challenge of managing wastewater in near-coastal tourist destinations around the Baltic Sea, particularly from small treatment plants (<2000 PE) that experience fluctuating flows due to seasonal tourism. These fluctuations make it difficult to meet environmental standards, potentially harming both the environment and tourism. The project has created a GIS-based inventory of small wastewater treatment plants within 100 km of the coast. This inventory includes crucial operational data like flow rates, pollutant levels, and treatment technologies. Initial findings reveal significant discrepancies in data management, regulations, and treatment standards across the Baltic Sea region countries, as EU legislation does not uniformly cover plants under 2000 PE. Key findings highlight that small treatment plants are often undocumented, their environmental impact underestimated, and regulations vary widely. Small plants can significantly contribute to nutrient pollution, affecting the Baltic Sea, particularly in local areas. The data gathered will support local authorities in identifying gaps and improving management strategies. This study stresses the need for harmonized data collection and reporting methods across countries and suggests establishing a unified database accessible to both specialists and the public. The status of the collected data depending on the type of data and country was as follows: 38.11% from Denmark, 46.14% from Estonia, 26.36% from Finland, 15.56% from Germany, 23.47% from Latvia, 34.77% from Lithuania, 14.51% from Poland, and 45.40% from Sweden. Ultimately, this project aims to enhance wastewater management, protect the environment, and improve tourist satisfaction in coastal regions.

A Review on Sludge Drying after High Pressure Filter Pressing

Sludge drying is a key step in municipal sludge treatment. After mechanical dewatering, the water content of sludge is usually between 50% and 70%, which needs further drying. The main drying technologies include thermal drying, solar drying, ultrasonic drying and microwave drying, among which thermal drying technology is the most mature. Research shows that low-temperature drying technology has obvious advantages in reducing the release of harmful substances, energy consumption and cost. In terms of technology application, domestic sludge drying process is mainly divided into direct drying and indirect drying. Direct drying directly evaporates water through high-temperature gas, while indirect drying makes use of heat conduction effect to contact with high-temperature media. At present, indirect drying technology is mainly used in China, such as paddle type, disc type and thin layer drying. In particular, the low-temperature heat pump drying technology excels in terms of safety, footprint and energy consumption, with its ability to operate at lower temperatures, reduce the release of hazardous gases, and achieve environmental and economic benefits in a closed-cycle mode. This technology is suitable for small and medium-sized wastewater treatment plants and shows clear advantages in cases of budgetary constraints.

Nature-based bioreactors: Tackling antibiotic resistance in urban wastewater treatment

The overuse and misuse of antibiotics have accelerated the selection of antibiotic-resistant bacteria, significantly impacting human, animal, and environmental health. As aquatic environments are vulnerable to antibiotic resistance, suitable management practices should be adopted to tackle this phenomenon. Here we show an effective, nature-based solution for reducing antibiotic resistance from actual wastewater. We utilize a bioreactor that relies on benthic (biofilms) and planktonic microbial communities to treat secondary effluent from a small urban wastewater treatment plant (<10,000 population equivalent). This treated effluent is eventually released into the local aquatic ecosystem. We observe high removal efficiency for genes that provide resistance to commonly used antibiotic families, as well as for mobile genetic elements that could potentially aid in their spread. Importantly, we notice a buildup of sulfonamide (sul1 and sul2) and tetracycline (tet(C), tet(G), and tetR) resistance genes specifically in biofilms. This advancement marks the initial step in considering this bioreactor as a nature-based, cost-effective tertiary treatment option for small UWWTPs facing antibiotic resistance challenges.

Sewage Derived Microplastic and Anthropogenic Fibre Retention by Integrated Constructed Wetlands

High loads of microplastics and anthropogenic fibres can be discharged from wastewater treatment plants (WWTPs) into surface water bodies. Integrated Constructed Wetlands (ICWs) are potentially well suited to provide a cost-effective mitigation solution at small WWTPs where conventional treatment is prohibitively expensive. This study aimed to assess the microplastic and anthropogenic fibre retention efficiency of two ICWs (Northrepps and Ingoldisthorpe) in Norfolk (UK) over a 12-month period (2022–2023). Analysing a total of 54 water and 23 sediment samples, the findings revealed that Northrepps ICW received on average 349,920 (± 763,776) anthropogenic fibres day−1, with a retention rate of 99.3%. No seasonal variation was observed in retention efficiency. Ingoldisthorpe ICW intermittently received anthropogenic fibres in low concentrations, with an average of 9504 (± 19,872) day−1 and a retention rate of 100%. Microplastics and anthropogenic fibres were prevalent in sediment samples of the first cell of Northrepps ICW, averaging 10,090 items kg−1 dry sediment, while none were found at concentrations above the limit of detection in the second or third cell. Of the 369 fibres analysed by ATR-FTIR, 55% were plastic (dominated by polyester). Of the 140 suspected microplastic fragments analysed by ATR-FTIR, 73% were confidently identified as plastic (mostly polystyrene, polyethylene, or polypropylene). This study demonstrates how ICWs can effectively retain sewage effluent derived microplastics and anthropogenic fibres. However, the accumulation of plastic waste in ICWs may complicate long term management and their cost-effectiveness.

Excess of Nutrients in Prefabricated or Compact Wastewater Treatment Plants: Review, Solution Alternative, and Modeling for Verification

Chile has numerous areas that lack sewage collection, including in the capital city. Sanitation in these cases is managed through individual solutions like septic tanks or small wastewater treatment plants (WWTPs) that use biological treatment, usually activated sludge with extended aeration. In general, the design of these systems adheres to the quality standards mandated by regulations for discharge, infiltration, or irrigation. In this scenario, traditional methods like increasing dissolved oxygen (DO) or hydraulic retention time (HRT) were unable to effectively reduce excessive nutrients. Therefore, literature related to nutrient excess and denitrification systems is consulted and reviewed to compile different solutions suitable for the presented issue. Potential solutions were modeled and verified using the free simulation software WRc STOAT. The software accurately predicted the unsatisfactory results of the current setup and provided parameters for the proposed modifications. Experience, precise user definition, influential characteristics, and modeling are essential in the design of WWTPs.

Anaerobic Digestion as a Possible Method of Managing Waste from Mushroom Production with Sewage Sludge as Co-Substrate

The mushroom agroindustry generates a huge amount of waste from mushroom production (WMP). The composition of WMP is not standardized but differs mainly in terms of organic matter (OM) content and OM biodegradability. This makes WMP management, including anaerobic digestion (AD), a significant challenge. A potential solution could be co-digestion of WMP with municipal sewage sludge (SS), especially SS generated in small rural wastewater treatment plants (WWTPs). Therefore, this study investigated mesophilic methane production (MP) from WMP, SS, and mixtures of SS and WMP at ratios of 70:30, 50:50, and 30:70 (w/w OM). Even though the maximum cumulative MP from WMP was relatively low (approx. 60 NL/kg OM), co-digesting WMP with SS increased both MP and the methane content of the biogas: with 30%, 50%, and 70% shares of SS, MP increased almost 2, 2.5, and 3.3 times, and the methane content increased to 61%, 62%, and 64%, respectively. As the SS content was increased, the kinetic coefficients of MP and OM removal decreased (from 0.211 to 0.146 d−1 and from 0.215 to 0.152 d−1), whereas the initial rate of MP and of OM removal increased (from 12.5 to 36.8 NL/(kg OM·d) and from 0.51 kg OM/(m3·d) to 0.59 kg OM/(m3·d), respectively). The effectiveness of OM removal (EOMrem) was lowest with WMP only, at 46.6%. When the SS content of the mixtures was increased to 30%, 50%, and 70%, EOMrem also increased to 55.3%, 60.1%, and 64.9%, respectively. The relationship between maximal MP and the overall OM removed was such that both increased simultaneously. The higher values of EOMrem and, consequently, the lower final contents of OM with more effective MP indicate that the organics were degraded more efficiently. These results suggest that co-digestion may be a profitable solution for simultaneously utilizing both of these waste products, increasing the efficiency of biogas production to such an extent that it would be profitable to conduct AD on mushroom farms. This is a flexible approach that allows varying proportions of WMP and SS to be used, depending on the availability of both substrates and the energy needs of the mushroom farm. However, it should be borne in mind that a higher share of WMP results in lower gas productivity.

Biochar from recovered cellulose as new admixture in constructed wetlands for micropollutant removal: A circular approach

This study aimed to evaluate the suitability of biochar produced by pyrolysis from recovered wastewater cellulose and activated biologically as an admixture in Constructed Wetlands (CWs) when applied as a post-treatment step to remove micropollutants (MPs) from municipal wastewater effluent. Two planted vertical flow mesocosm CWs with cellulose-based admixtures of different origins (plant residue and recovered toilet paper) were fed with a municipal wastewater effluent representative for rural catchments. The results showed an average MPs elimination of 89.1 % for the activated biochar produced from recovered cellulose when 15 relevant compounds are considered and a reduction of the risk from compounds cocktail below the maximum acceptable level having diclofenac, carbamazepine, PFOS, ciprofloxacin and clarithromycin as main risk drivers (Risk Quotient > 1). The implementation of a circular approach to reduce MPs was finally conducted for the Blies catchment (Saarland region in Germany) characterized by low population density and small, sensitive water bodies. This approach demonstrates the feasibility of combining cellulose recovery with a fine sieve in large wastewater treatment plants (WWTPs) and providing biochar produced from recovered cellulose as an admixture to small WWTP where CW is an affordable solution for MP mitigation.

Chronic shedding of a SARS-CoV-2 Alpha variant in wastewater

BackgroundCentral Michigan University (CMU) participated in a state-wide SARS-CoV-2 wastewater monitoring program since 2021. Wastewater samples were collected from on-campus sites and nine off-campus wastewater treatment plants servicing small metropolitan and rural communities. SARS-CoV-2 genome copies were quantified using droplet digital PCR and results were reported to the health department.ResultsOne rural, off-campus site consistently produced higher concentrations of SARS-CoV-2 genome copies. Samples from this site were sequenced and contained predominately a derivative of Alpha variant lineage B.1.1.7, detected from fall 2021 through summer 2023. Mutational analysis of reconstructed genes revealed divergence from the Alpha variant lineage sequence over time, including numerous mutations in the Spike RBD and NTD.ConclusionsWe discuss the possibility that a chronic SARS-CoV-2 infection accumulated adaptive mutations that promoted long-term infection. This study reveals that small wastewater treatment plants can enhance resolution of rare events and facilitate reconstruction of viral genomes due to the relative lack of contaminating sequences.

Efficient Technologies Of Small Wastewater Treatment Plants

This article discusses the results of a scientific research work on the use of various types of small wastewater treatment plants for objects of various purposes in conditions of periodic use. The study was carried out on model and real wastewater with subsequent refinement of the characteristics using the calculation methods of the DWA standard.The results of the operation of laboratory facilities operated in the restart mode after a certain time are presented. As a result, the duration of restoration of the operating mode under periodic operating mode was considered for each plant under study and the effectiveness of biological wastewater treatment was determined. More promising technological solutions for small wastewater treatment plants under intermittent use conditions have been identified. According to the sum of the characteristics, the operation of the flooded biofilter system, which is capable of entering the operating mode within 5 days after a long shutdown, is highlighted. These technologies are recommended for use in resort areas and hotels.

Minimising greenhouse gas emissions from small and medium-sized wastewater treatment plants

Minimising greenhouse gas emissions from small and medium-sized wastewater treatment plants

SARS-CoV-2 Wastewater Monitoring in Thuringia, Germany: Analytical Aspects and Normalization of Results

Wastewater monitoring for SARS-CoV-2 is a valuable tool for surveillance in public health. However, reliable analytical methods and appropriate approaches for the normalization of results are important requirements for implementing state-wide monitoring programs. In times of insufficient case reporting, the evaluation of wastewater data is challenging. Between December 2021 and July 2022, we analyzed 646 samples from 23 WWTPs in Thuringia, Germany. We investigated the performance of a direct capture-based method for RNA extraction (4S-method) and evaluated four normalization methods (NH4-N, COD, Ntot, and PMMoV) in a pooled analysis using different epidemiological metrics. The performance requirements of the 4S method were well met. The method could be successfully applied to implement a state-wide wastewater monitoring program including a large number of medium and small wastewater treatment plants (&lt;100,000 p.e) in high spatial density. Correlations between wastewater data and 7-day incidence or 7-day-hospitalization incidence were strong and independent from the normalization method. For the test positivity rate, PMMoV-normalized data showed a better correlation than data normalized with chemical markers. In times of low testing frequency and insufficient case reporting, 7-day-incidence data might become less reliable. Alternative epidemiological metrics like hospital admissions and test positivity data are increasingly important for evaluating wastewater monitoring data and normalization methods. Furthermore, future studies need to address the variance in biological replicates of wastewater.

Urban wastewater disinfection by iron chelates mediated solar photo-Fenton: Effects on seven pathogens and antibiotic resistance transfer potential

The effects of solar photo-Fenton (SPF) process mediated by the iron chelate Fe3+ imminodisuccinic acid (Fe:IDS) on both the inactivation of seven relevant pathogens and the potential for antibiotic resistance transfer (degradation of antibiotic resistance genes (ARGs) and after treatment regrowth), in real secondary treated urban wastewater, were investigated for the first time. A comparison with results obtained by sunlight/H2O2 process and Fe3+ ethylenediaminedisuccinic acid (Fe:EDDS) SPF was also carried out. ARGs were quantified by polymerase chain reaction (PCR) in samples before and after (3 h) the treatment. The persistence of the selected pathogens and ARGs was also evaluated in regrowth tests (72 h) under environmentally mimicking conditions. Fe:IDS SPF resulted to be more effective (from 1.4 log removal for Staphylococcus spp. to 4.3 log removal for Escherichia coli) than Fe:EDDS SPF (from 0.8 log removal for Pseudomonas aeruginosa to 2.0 log removal for Total coliphages) and sunlight/H2O2 (from 1.2 log removal for Clostridium perfringens to 3.3 log removal for E. coli) processes for the seven pathogens investigated. Potential pathogens regrowth was also severely affected, as no substantial regrowth was observed, both in presence and absence of catalase. A similar trend was observed for ARGs removal too (until 0.001 fold change expression for qnrS after 3 h). However, a poor effect and a slight increase in fold change was observed after treatment especially for gyrA, mefA and intl1. Overall, the effect of the investigated processes on ARGs was found to be ARG dependent. Noteworthy, coliphages can regrow after sunlight/H2O2 treatment unlike SPF processes, increasing the risk of antibiotic resistance transfer by transduction mechanism. In conclusion, Fe:IDS SPF is an attractive solution for tertiary treatment of urban wastewater in small wastewater treatment plants as it can provide effective disinfection and a higher protection against antibiotic resistance transfer than the other investigated processes.

The occurrence of heavy metals and antimicrobials in sewage sludge and their predicted risk to soil — Is there anything to fear?

The study assessed the occurrence of legally-monitored heavy metals and unmonitored antimicrobials in sludge from small, medium, large and very large municipal wastewater treatment plants (WWTPs), and the predicted environmental risk and risk of resistance selection associated with sludge administration to soil. The temporal variations of the studied compounds in sludge and associated risks to soil were determined by sampling over a year.Although the highest concentrations of heavy metals were noted in sludge from the largest WWTP, i.e. from 1.50 mg/kg (mean 1.61 mg/kg) for Cd to 2188 mg/kg (mean 1332 mg/kg) for Zn, the obtained values only reached a few percent of the legal limits. The same WWTP also demonstrated lower concentrations of antimicrobials compared to the smaller ones. The highest concentrations of antimicrobials, ranging from 24.04 μg/kg for trimethoprim to 900.24 μg/kg for tetracycline, were found in the small and medium WWTPs. However, due to lack of regulations at the national and EU levels, the results cannot be compared with legal limits. Principal Component Analysis (PCA), cluster and heatmap analysis separated samples according to WWTP size. Small WWTP demonstrated correlation with antimicrobials (tetracycline, trimethoprim, clindamycin, ciprofloxacin and ofloxacin), while the large and very large WWTP revealed correlations with heavy metals (Cu and Cr).The obtained environmental risk quotients confirmed that the heavy metals did not present a threat, measured either as individual risk quotients (RQenv), cumulative risk (RQcumulative) or risk of mixture of heavy metals (RQmix-metals). In the case of antimicrobials, only tetracycline demonstrated moderate RQenv, RQcumulative and RQmix-antimicrobials in the small WWTP sludge, with values of 0.1 to 1. Our findings highlight the importance of monitoring sewage sludge before soil application, especially from small WWTPs, to reduce the potential environmental impact of antimicrobials. They also confirm our previous data regarding the environmental risk associated with various toxic compounds, including emerging contaminants, in the sludge from small WWTPs.

Combining chemical, bioanalytical and predictive tools to assess persistence, seasonality, and sporadic releases of organic micropollutants within the urban water cycle

Using a novel liquid chromatography-tandem mass spectrometry method with large volume direct injection and quantitation via isotope dilution, we evaluated the presence of 55 organic micropollutants in wastewater effluents, and locations within the Bow River and Elbow River watersheds in and around the city of Calgary, Alberta, Canada. In addition to establishing baseline micropollutant data for water utility operations, our study aimed to enhance our understanding of micropollutant behavior in the urban water cycle, assess the contributions of three wastewater treatment plants (WWTPs) to downstream receiving waters, explain the potential causes of total estrogenicity measured using the yeast-estrogen screen assay (YES), and prioritize a subset of substances for continuous monitoring. With data spanning 48 months and 95 river km, our results indicate the extensive persistence of metformin (antidiabetic), seasonality of N,N‑diethyl-m-toluamide (DEET, insect repellant), O-desmethylvenlafaxine (antidepressant metabolite), and sulfamethoxazole (antibiotic) in source waters, and sporadic detections of a well-known perfluoroalkyl substance (PFOA). The seasonality of pharmaceuticals at the sentinel downstream monitoring site appeared to coincide with river dilution while that of DEET was likely attributable to peak usage during the warmer months. Steroidal estrogens were rarely detected in wastewater effluents although total estrogenicity via YES was evident, suggesting the presence of less potent but more abundant non-steroidal estrogens (e.g., flame retardants, bisphenols, and phthalates). A conservative mass balance analysis suggests that the largest WWTP (serving a population of >1 million) consistently contributed the highest load of micropollutants, with the exception of metformin, which appeared to be influenced by a smaller WWTP (serving 115,000) that operates a different activated sludge process. We consider metformin, sucralose, diclofenac, and venlafaxine as more effective conservative tracers of wastewater pollution due to their notably higher concentrations and persistence in the Bow River compared to carbamazepine and caffeine, respectively. Finally, hierarchical clustering revealed a close association between E. coli and caffeine, supporting the use of caffeine as an indicator of short-term, untreated anthropogenic inputs. Overall, this study yields valuable insights on the presence, behavior, and sources of organic micropollutants in the urban water cycle and identifies indicators of anthropogenic impacts that are useful for prioritizing future monitoring campaigns in Calgary and elsewhere.

Saving Energy in Biological Wastewater Treatment by Using Extremely Low-Frequency Electric Field—Pilot-Scale Study

The results of a pilot-scale study on the influence of electric field use for stimulating the active sludge in the biological purification tank of a small capacity wastewater treatment plant (up to 600 m3/day) are presented. Through specific comparative chemical tests (DO, COD, N-NH4, and Pt) it was found that, by applying a sinusoidal electric field of 5 Vrms/m at 49.9 Hz on the active sludge suspension, the overall pollutant denitrification process speed is doubled compared with the reference case when no stimulation is used. Also, under identical operating conditions, the residual pollutant content of the biological treatment tank outlet water is reduced approximately three times for COD and approximately two times for N-NH4 and Pt compared to the reference tank. These findings lead to the conclusion that, by stimulating the active sludge microbial activity of the wastewater treatment plants by a sinusoidal electric field of 5 Vrms/m at 49.9 Hz, the time of the biological purification treatment can be reduced by approx. 50%. This leads to a corresponding decrease in energy consumption, which usually represents more than 30% of a wastewater treatment plant’s specific electricity consumption.

Meandering Flow Filter for Phosphorus Removal as a Component of Small Wastewater Treatment Plants—A Case Study

Chemical phosphorus removal in wastewater treatment plants can be carried out by precipitation with iron or aluminum salts or by filtering wastewater through a bed of active sorptive material. This work aimed to investigate whether using a meander flow filter filled with the sorption material Rockfos® would improve phosphorus removal efficiency in a selected wastewater treatment plant. Tests were performed under laboratory conditions using a model of a meander flow filter and a similar filter under field conditions at full technical scale. This filter was the final element of a hybrid constructed wetland located in the village of Białka in the municipality of Dębowa Kłoda in southeastern Poland. A laboratory model of a phosphorus removal filter with vertical incomplete baffles forcing a meandering water flow was constructed to determine the hydraulic conditions of the flow. After one year of operation, the filter with horizontal wastewater flow operating at its full technical scale (without meanders) was modified by inserting appropriate baffles that were analogous to those in the laboratory model. The analysis of the hydraulic conditions in the laboratory model showed that, under the assumed conditions, wastewater flows through the filter layer in a laminar motion, so such filters can be modeled using the Kozeny–Carman formula. It was shown that, after approximately a year of operation in a filter operating at full technical scale, before modification, dead spaces formed, thus causing the channel and primarily surface flow of wastewater. The phosphorus removal efficiency during this test period averaged 9.4%. After introducing baffles and forcing meander flow in the filter chamber, the efficiency increased to 40.6%. The results indicate that meander flow filters can improve phosphorus removal efficiency in small wastewater treatment plants.

Centralization of wastewater treatment in a tourist area: A comparative LCA considering the impact of seasonal changes

Nowadays, environmental protection has become a topic of primary importance, and the interest in wastewater treatment plants (WWTPs) has increased due to the need for a paradigm shift from linear to circular economy. The centralization level of wastewater infrastructure is the basis for a successful system. The aim of this study was to investigate the environmental impacts generated from the centralized treatment of wastewater in a tourist area in central Italy. The combined use of BioWin 6.2 simulation software and life cycle assessment (LCA) methodology was implemented to evaluate the potential connection of a small decentralized WWTP to a medium-size centralized facility. Two different scenarios (decentralized system, corresponding to the current situation, and centralized) were evaluated in two separate periods: high season (HS), corresponding to the main tourist season, and low season (LS), which is the period before the main tourist season. Two sensitivity analyses were conducted, assuming different N2O emission factors, and considering the period at the end of tourist season, respectively. Although with modest advantages (up to −6 % in pollutant emissions), WWTP connection was the best management option in 10 out of 11 indicators in HS, and 6 out of 11 categories in LS. The study showed that wastewater centralization was promoted by scale factors in HS, as the most impactful consumptions decreased as the degree of centralization increased; on the other hand, the decentralized system was less penalized in LS, as small WWTP was less stressed and energy consuming in this period. Sensitivity analysis confirmed the results obtained. Site-specific conditions can lead to conflicting circumstances, as key parameters may have different behaviors depending on seasonal variations, and the degree of centralization in tourist areas should be addressed by distinguishing separate periods, based on changes in tourist flows and pollution loads.

Avaliação de desempenho de uma estação de tratamento de esgoto movida a energia solar (SBR de dois estágios) operada em regiões de clima tropical

ABSTRACT This article investigates the performance behaviour of a small decentralized wastewater treatment plant with a capacity of up to 50 population equivalents powered by solar energy. The two-stage sequencing batch reactor (SBR) consists of a photovoltaic (PV) system to deliver energy, a battery storage for night operation and two reactor tanks. In the experimental period of 157 days, the wastewater inflow was increased from 3 to 4.5 and 6 m3.d−1, with sludge ages of 32 ± 2 d in the beginning and 21 ± 2 d in the end. The results corresponding to the different phases indicated high efficiency and stability of the system with domestic wastewater, reaching efficiencies of the test periods of 93 ± 2%, 86 ± 4% and 93 ± 6% for removal of chemical oxygen demand, nitrogen and phosphorus, respectively. In the last 51 days, aeration in the night was interrupted for three hours to save energy and study the behaviour of extended non-aerated phases. A shortening of the aeration phases can help to extend the lifetime of batteries and reduce operational costs, while limiting values in the outlet are still met.

Effects of a point source of phosphorus on the arsenic mobility and transport in a small fluvial system

One of the leading causes of As release from streambed sediments into freshwater systems is competition with phosphate. Among important sources of P to the fluvial ecosystems are wastewater treatment plants (WWTP), estimated to account for 25–45% of all P in surface waters. In this paper, long-term effects of discharged phosphorus from a small WWTP on the arsenic mobility were studied in an As-enriched fluvial system (approx. 240 mg/kg) in central Czech Republic. After 7 years of elevated P (≤7.7 mg/L) in the stream water, the total As decreased by 25% and the total P increased by 40% in the sediments downstream (at a distance of 66 m). The results of the chemical extractions and mineralogical analyses indicated that the changes in the concentration were mostly due to the sorption processes in the Fe (oxyhydr)oxides (goethite and hematite). In the downstream samples, the As in these phases decreased two-fold, and P was significantly enriched by 45–140%. Phosphorus was also found precipitated as newly formed Ca phosphates. The stream water monitoring indicated that the discharged P was either sequestered when the levels of dissolved P were high (>2.3 mg/L) or released from the downstream sediments when these levels were low (<∼1.5 mg/L). Meanwhile, As was continuously mobilized from the downstream sediments likely due to (i) the ongoing As desorption from the exterior parts of the Fe (oxyhydr)oxides at high aqueous P levels and (ii) the dissolution of As-bearing Ca phosphates at low dissolved P levels. These findings clearly demonstrate that point sources of P to streams and rivers, such as WWTP, may result in the permanent and long-term release of As from contaminated streambed sediments.

Evaluating the Performance of Small Wastewater Treatment Plants

An evaluation of the treatment performance of small wastewater treatment plants with the capacity of ≤50 PE (population equivalent) was carried out using data collected in external monitoring reports, taking Upper Austria as an example. External monitoring data for 2009–2018 were available for this analysis. About 2′500 small WWTPs are in operation in Upper Austria. The main technologies implemented include about 870 Sequencing Batch Reactor (SBR) plants, 630 conventional activated sludge (CAS) plants and 535 vertical flow (VF) wetlands. The data set for the present evaluation comprises a total of approx. 14′000 external monitoring reports from approx. 2′000 small WWTPs. This paper presents the results of this evaluation. The evaluation showed that all technologies meet the legal requirements regarding BOD5, COD and NH4-N effluent concentrations. However, for small WWTPs with a polishing stage consisting of a VF wetland, the median values of the effluent concentrations and the limit value exceedances are significantly lower. There is no evidence of a significant deterioration in treatment performance with the age of the plant for any technology. However, for all technical WWTPs such as SBR and CAS plants, a greater fluctuation of the measured values (i.e. the median values of the effluent concentrations) can be observed when the plant is longer in operation. Compared to technical WWTPs, this fluctuation is significantly smaller in VF wetlands.