Water bodies' quality objectives are defined in accordance with the Urban Wastewater Treatment Directive 91/271/EEC and the Water Framework Directive 2000/60/EC. For regulation and control of small-sized waste-water treatment plants (WWTPs), responsibility is delegated in Italy to Regional Authorities that fix specific regulations (Water Protection Plan WPP included in the River Basin Management Plan RBMP) in collaboration with the District Authorities. Small (<2,000 population equivalent – PE) and medium sized (2,000–10,000 PE) WWTPs in the Veneto Region (North Italy) represent about 10% of the total organic load (Imhoff systems included). This also comprises some industrial discharges. Due to the urban sprawl, plants are spread over the regional territory. In the Veneto Region, data from the official census reveals there are n. 248 plants under 2,000 PE and 135 plants in 2,000–10,000 range while the total number of authorized plants is 488 for a total potentiality of 9,141,572 PE. Data from institutional controls performed by the Veneto Regional Environmental Agency (ARPAV) on WWTPs has been recovered for all the WWTPs with up to 10,000 PE in the provinces of Venice, Treviso and Vicenza (for a total of 306,118 PE and for a total of 164 plants) in the period 2008–2015 and elaborated to assess critical parameters and plants. The general situation, critical issues and case studies have been presented and discussed. Organic load, nutrients and Escherichia coli are the most critical parameters considering the regional WPP.

Water appears to be a very precious and scarce resource due to population growth, increasing human needs (Leverenz et al. 2011) and climatic changes. Water scarcity is one of the biggest challenges humanity is facing. Approximately 2.5 billion people – 1/3 of the world's population – live in areas that the World Bank characterizes as ‘water stressed’; in 15 years' time, this will concern 2/3 of the world's population (Organica 2014). At the European level, environmental objectives are fixed by Directive 271/91/EEC (wastewater treatment Directive WWTD; EC, 1991). It requires that all the agglomerations with a higher than 2,000 population equivalent (PE) be served with sewerage systems and adequate wastewater treatment plants (WWTPs) (primary/secondary treatment), while Directive 2000/60/EC (WFD; EC, 2000) requires all significant water bodies for internal waters to achieve (or maintain) a Good Ecological Status within 2015. Moreover, in sensitive areas, specific limit values must be applied to discharges of agglomerations above 10,000 PE.

The WFD establishes that all point and non-point emission sources allowed into surface waters must be controlled on the basis of a combined approach (art. 10) including controls on emissions based on the best available techniques (BTA), controls on the emission limit values and application of the best environmental practices for diffuse impacts. To characterise significant pollution from anthropogenic points and diffuse pressure sources it is necessary to investigate pollution caused by substances in Annex VIII of the WFD (list of main pollutants). For these purposes, an inventory analysis of industrial cycles and diffuse pollution sources is fundamental. The combined approach needed to reduce pollution loads (organic, nutrients, dangerous substances, etc.) is based on the establishment of limit values for discharges and quality standards for the receiving water bodies. It is defined by analysis of the major pressures and impacts on the receiving water basin, in order to decide the type of interventions necessary to guarantee quality objectives and to prioritise interventions (Benedetti et al. 2008). With the WFD the water protection approach has been developed considering the effective maximum load capacity of aquatic ecosystems to guarantee the presence of large and diversified communities, through qualitative and quantitative protection of waters. Ecological and chemical status must be determined with the implementation of specific monitoring programmes.

Small plants have always received greater attention with reference to urbanisation processes and the sprawl of anthropogenic activities and settlements (Jefferson et al. 2000). From Directive 271/91/EEC we can deduce that small plants include all the WWTPs that collect wastewater from agglomerations with lower than 2,000 PE. Different indications are presented by different Italian authors (Libralato et al. 2012): different values are considered such as 2,500 PE or 5,000 PE. For the present analysis, small-sized WWTPs are considered to be plants up to 2,000 PE in accordance with Directive 271/91/EEC, while plants in the 2,000–10,000 PE range are referred to as medium-sized plants. Directive 271/91/EEC does not supply specific indications for agglomerations below 2,000 PE. On the one hand, identifying the agglomeration is a key issue directly correlated to the obligations which need to be respected when dealing with discharge limit values and sewer requirements. On the other hand, sewer realization must consider the feasibility of installing sewers both from the technical and financial point of view. The tendency over the last few years has favoured centralisation, especially in the urban context in developed countries. Nowadays there is sufficient evidence to suggest that decentralisation is often more effective in complying with water quality objectives and with economic and technical constraints. The realization of small-sized treatment plants can reduce the discharge dilution phenomena, maximise treated wastewater reuse and the recovery of by-products (Libralato et al. 2012). Research conducted on small-sized plants at National level has been performed, in Italy, by the regional environmental agencies; from the study more than 9,000 plants below 2,000 PE and more than 1,500 in the 2,000–10,000 PE range have been registered at national level (Falletti et al. 2010). Despite the lower load treated, the small and medium-sized WWTPs appear both important and crucial in determining water bodies' quality because of their spatial distribution and especially with regard to their role in treating local loads which cannot be collected to large plants due to technical aspects and/or excessive collection costs. Small-WWTPs also appear to be particularly important in safeguarding delicate and high human pressure environments in coastal areas (Diaz et al. 2016).

Italian Regulations governing the control of limit values for WWTPs are represented by the Italian Decree n. 152/2006, which transposed WWT and WF directives into the Italian legal framework and which fixes limit values for plants higher than 2,000 PE. To achieve the environmental objectives of water bodies, specific limit values for discharges must be defined through River Basin Management Plans (RBMPs). In Italy, the RBMPs were set up thanks to the regional Water Protection Plans (WPPs) developed by the Regions. Within the activities of public environmental institutions the management of the small and medium sized public WWTPs needs to consider the effective risk, in real terms, carried by the type of wastewater received and treated as well as the specific discharge limits to be satisfied referred to in the RBMP and WPP. According to Italian Law (Decree n. 152/2006), the planning and regulation of small-sized plants fall under the responsibility of the Regional Authorities. Environmental objectives of significant water bodies are defined based on the RBMP. Environmental Quality Standards (EQS) are guaranteed with the establishment of specific limit values. Whereas national regulations in Italy fix limit values for agglomerations higher than 2,000 PE, the limit values for smaller agglomerations are defined by the Regional Authorities. For small and medium-sized WWTPs one of the most critical issues is the choice of the most appropriate treatment technology. Environmental and economic performance assessment was been suggested by Benedetti et al. (2008).

In the area of the study (Veneto Region, North-East Italy) the WWTPs considered are based on the activated sludge process (ASP) and the infrastructure generally consists (Hreiz et al. 2015) of: an aerated bio-reactor; a settling tank; a sludge recycle line of settled sludge to the bio-reactor. Based on the sludge, retention time (sludge age) of 4–30 days guarantees good floc sedimentation (Hreiz et al. 2015). The population of the Veneto Region (Figure 1) is 4,927,596 (Veneto Region 2016b) and its distribution across its seven provinces is reported in Figure 2. This study presents and discusses analytical data regarding the characterization of the final discharges elaborated by the control Authority for small and medium sized WWTPs with a potentiality of up to 10,000 PE for the provinces of Venice (858,198 Inhabitants in 2014), Treviso (887,293 Inhabitants) and Vicenza (869,718 Inhabitants). Together, these three provinces represent 53.1% of the total population of the Veneto Region (4,927,596 inhabitants in 2014). The Veneto Region has established different limit values for WWTPs based on the vulnerability of the area concerned (lower limits in groundwater recharge areas, drainage area close to sea or lakes, etc.). A more effective approach for the control of WWTPs with PE below 2,000, which considers the real environmental risks, has been developed by the Veneto Regional Environmental Prevention and Protection Agency (ARPAV). This is the institutional body responsible for environmental monitoring and controls and developed this approach following technical assessment based on and specific know how gained from more than 10 years of control data collection and expert analysis. A hierarchical approach has been proposed and applied (Ostoich et al. 2010), supported by specific protocol and functional verification. The most critical parameters are organic load, nutrients and microbiological parameter (Escherichia coli).

Identification of agglomerations

To guarantee satisfactory load reduction to protect water resources, wastewater collection, organic and nutrient load treatment have been regulated by Directive 271/91/EEC. The directive requires that collection is realized for each agglomeration with a PE above 2,000; for smaller centres or for isolated dwellings, individual treatment systems are allowed. The directive (art. 2) defines an ‘agglomeration’ as an area where the population and/or economic activities are sufficiently concentrated for urban wastewater to be collected and conducted to an urban WWTP or to a final discharge point. The identification and characterization of the agglomerations according to Directive 91/271/EEC must guarantee a satisfactory level of treatment (‘appropriate treatment’) for urban wastewaters and the achievement of the quality objectives for water bodies as established by Directive 2000/60/EC. The directive aims to prevent the environment from being adversely affected by the disposal of insufficiently-treated urban wastewater, and indicates the general need for secondary treatment of urban wastewater; in sensitive areas (to be identified according to criteria indicated in the same directive) it prescribes more stringent treatment; whereas in less sensitive areas primary treatment can be considered appropriate.

The existence of an agglomeration is neither dependent on the existence of a wastewater collection system nor of a treatment plant. Therefore ‘agglomeration’ can also indicate areas with low urban population density, but where a collection system does not yet exist and/or where wastewaters are treated in individual systems or in other alternative systems. The agglomeration is the identified area in which integrated urban wastewater management, as part of the integrated water cycle, is applied (Benedetti et al. 2008; Diaz et al. 2016) in order to guarantee effective wastewater collection, adequate treatment and final discharge which respects specific limit values and consequently achieves a good ecological status in the receiving water bodies as established by the WFD. Criteria for the identification of an agglomeration have been presented and discussed by Ostoich & Carcereri (2011). For the Veneto Region, the identification of agglomerations has been established by the Regional Executive Board Resolution n. 1955/2015 (Veneto Region 2015): 729 agglomerations have been identified in the whole regional territory.

Discharge limit values required to achieve The Good Ecological Status of water bodies

The Italian Decree n. 152/2006 in accordance with the WWTD and the WFD establishes (art. 124) that all discharge points must be authorized; technical directives are reported in Annex 5 of the Decree, together with limit values for public WWTPs and industrial discharges into rivers, lakes, sea, ground-waters, soil, etc. Discharge sampling must be executed over a 24 hour period (mean weighted sample, function of flow). Specific limit values are reported for BOD5, COD and SS parameters (tab. 1-Annex 5 Decree – see Table 1) and for nutrients Ntot and Ptot typical of domestic wastewaters. For other parameters such as industrial wastewaters and mixed wastewater, a specific list of pollutants has been established (tab. 3-Annex 5 Decree). Microbiological pollution is traced with the Escherichia coli parameter for which only a suggested limit value is indicated. This, gives the control Authority flexibility when prescribing specific values in order to achieve the required quality of the water body based on its effective use. The Decree reports the control frequency for the parameters in tab. 12 and in tab. 3-Annex 5 as in the following Tables 2 and 3. The WWTP manager must carry out an equivalent number of self-monitoring assessments with the same criteria (weighted for 24 hours) and is obliged to divulge the results, should there be a formal inspection by the control authority. Ntot and Ptot limits can be assessed only on end-of-pipe concentration or on flows in/out of the plant for abatement efficiency. It is important to observe that limit values for discharges are established for ‘agglomerations’ where PE is reported; if there is one or more plants in the same agglomeration, all of them must respect this limit. Therefore it is clear that the identification of the agglomerations is fundamental for rational and appropriate waste-water treatment.

For WWTPs in the Veneto Region with potentiality below 2,000 and with reference to the Water Protection Plan (WPP; Veneto Region 2009) and threshold inhabitant values (S) higher than that specified according to the area vulnerability the control frequency is established as in Table 2. In the range S-2,000 PE, the discharge control frequency must be respected as long as plant PE is not below S. Table 4 lists the homogeneous zones identified by the WPP according to their different vulnerabilities and fixes different threshold values (S) for each one (i.e. homogeneous zones: mountain zones; aquifer recharge areas and the Venice lagoon catchment area; plane areas with high population density; plane areas with low population density; coastal zones). Limit values based on Italian Decree n. 152/2006 and on the WPP for the Veneto Region for the parameters being considered (chosen according to significant data) in the present study are reported in Table 5. Unless the regional WPP requires different limit values to be met for urban WWTPs, due to the vulnerability of the area of discharge, in this study identical precautionary limit values for each plant with reference to column C (Table 5) have been considered in discharge data elaboration for the sole purpose of indicative assessment.

Integrated control approach for WWTPs and control delegation protocol

The control of WWTPs has multiple aims: it is useful to verify conformity with emission limits, to quantify technical performance and for environmental auditing performed by the plant manager. This control must consider emissions and the consumption of resources (matter, energy, exceptional emissions during transitional phases like start up, stopping, etc.) and fugitive emissions must also be assessed. The approach to environmental controls developed and applied by ARPAV since 2000 has been inspired by the hierarchical principle, derived from Italian regulations (see Tables 2 and 3 for discharge control frequency) and from a risk assessment of the different pressure sources. This is a preventive, integrated approach, or more specifically, a control approach where the aim is not to verify just one environmental aspect (such as analytical control), but to gather all data and information which are ‘diagnostic’ for the assessment of the functioning of the plant (point pressure source).

The developed approach is based on the following controls (Ostoich et al. 2010): Documentary (textual verification without measurements, sampling and/or analysis undertaken by the plant manager); Technical (verification of structural characteristics of the plant and its accessories with regard to EQS); Management (verification of management requirements of the plant, verification of self-certifications, audit of the environmental management system); Analytical (direct monitoring of the environmental impact aimed at guaranteeing compliance with pertinent environmental limits).

An operative protocol for WWTP control has been developed and applied on the basis of the above criteria. The protocol aims to answer the following questions: What should be controlled? In which conditions should the control be conducted? With what frequency should controls be carried out? As established by Annex 5 of Italian Decree n. 152/2006, part three, it is possible to delegate part of the controls to the plant manager only if he guarantees that specific control protocol is carried out and the resulting data is transmitted to the control Authority. An approach for discharge control delegation has been applied and monitored for many years and has been proposed by ARPAV to the Veneto region. The proposed criteria was approved by Veneto Region (2011) with decision 578/2011. These criteria take into consideration the historical knowledge gathered by the control authority concerning the behaviour of a specific plant, based on analytic controls performed over time. In addition they also consider the specific characteristics of the territory (industries connected to the sewer), known problems and critical issues including plant reliability for the observance of limit values, good structural functionality of the plant (the plant adequately dimensioned for the hydraulic and organic load it receives), ordinary use of a support laboratory with quality assurance warranty, existence of a data transmission system from plant manager to control authority approved by the latter. Functionality verification integrates the above mentioned control protocol (Ostoich et al. 2010). Moreover the use of the Sludge Biotic Index (Madoni 1994) could support integrated controls of small and medium sized plants, by reducing analytical activities but also by improving knowledge of plant efficiency and reliability to respect discharge limit values.

Monitoring and control data management systems in the Veneto Region

Data from institutional monitoring and controls, carried out by the Regional Environmental Agency on WWTP discharges, have been extracted from the Veneto's Regional Environmental Information System (SIRAV). The chemo-physical, chemical and biological data produced by the laboratories of the regional Environmental Agency are stored on local database systems (LIMS – Laboratory Information Management System) after a two-phase control process and eventually converge into the SIRAV system.

Sampling and analytical methods for chemical parameters

Official sampling and analytical methods adopted in Italy were applied during this study: Analytical methods (APAT 2003) used since 2004. Where analytical methods were lacking in the Italian national legal framework, international official methods were also used (i.e. APHA et al. 1998). The sampling techniques were the following: mean-composite sampling on a 24 hour basis for WWTP effluents (in accordance with Directive 91/271/EEC) for all the parameters except for microbiological ones for which instantaneous sampling was performed. In this study when measured data are below the Limit of Quantification (LOQ) of the analytical methodology followed, the value considered for assessment applies the same LOQ (precautionary approach).

The census of WWTPs and Imhoff systems in the Veneto Region

The authorized WWTPs and Imhoff treatment systems have been taken from the official census (Regional Cadaster managed by ARPAV) and are reported in Table 6. In Table 7 and Figure 3 the project potentiality for each treatment system is reported. Comparison with the potentiality determined in 2010 (8,896,020 PE; Ostoich & Carcereri 2011) shows an increase of 433,523 PE. There are 240 WWTPs above PE 2,000 with a total potentiality of 8,954,509 PE. Of these plants, 135 are between 2,000 and 10,000 PE, 72 between 10,000 and 50,000 PE and 33 are above 50,000 PE. The total number of WWTPs below PE 2,000 are 248, with a total capacity of 187,063 PE, which represents about 2% of the total treatment potentiality in the Veneto region, while Imhoff systems with a potentiality of 187,971 PE account for an additional 2%. If we consider Imhoff systems, WWTPs <2,000 PE together with those <10,000 PE, the total potentiality is about 10% of total PE.

The controls on WWTPs discharges in the Veneto Region

A heterogeneous result comes to light following the assessment of control data concerning the chosen WWTP samples carried out between 2008 and 2015 and based on the typology of assessment (documentary, technical, management, analytic) and characteristics of the province. It must be noted that data assessment of individual typologies has been performed only partially due to the lack of detailed data regarding the control categories for the years 2014 and 2015, while the total number of controls for each province is available. Based on the number of the total controls performed, the situation appears heterogeneous both at regional and provincial levels.

This aspect probably depends on the different control policies required by each of the 7 provinces in the Veneto region, on the specific needs of the provincial Departments of the Regional Environmental Agency but also on the diverse mix of economic activities and, consequently, the industrial infrastructures in the provinces. Figure 4 reports the total number of controls performed on WWTPs in all the provinces in the period 2008–15.

With reference to the analysis of data elaborated for each typology of control, with the exception of the period 2014–15, the situation appears even more heterogeneous. This variability can be justified if we consider the different plants in each of the provinces, but it could also be correlated to the different objectives set each year by the provincial departments of the Agency, based on the availability of resources but also on the needs of the individual provinces. In Figure 5(a)5(d) documentary, technical, management and analytic controls for WWTPs in the Veneto region (all seven provinces) are reported as total cumulative value and per year (2008–13, no data are available for 2014 and 2015).

During data elaboration, mean values and std dev were calculated for each control typology, divided by year and province. This produces a more accurate comparison by reducing differences linked to the diverse plant infrastructure available in each province and data on controls have been normalized for the existing plants identified in the official census for each province and for each year (in Figure 6(a)(d) weighted controls are reported).

It is important to note that multiple controls have been carried out. They include analytical controls on discharges and documentary controls to assess to what extent environmental permitting regulations have been respected. Technical and management controls have also been performed. It is evident that in the case of the Vicenza province, the higher total controls is correlated to the larger number of WWTPs to be assessed, as well as the higher potentiality. However, if the number of controls performed and the number of WWTPs assessed for each province (weighted controls) are considered, in some provinces the number of weighted controls always appears greater than the mean value as is the case in the Province of Padova. This is due to to the fact that they are organized differently and the set of assessments required by the local Authorities also differs. If we analyse the individual controls subdivided by typology and weighted per number of WWTPs in each province, without classifying them, the situation appears more or less heterogeneous. For example, the number of weighted controls in some provincial departments (in particular that of Padova), appears higher than the mean annual value and, in some instances, higher than the total value referred to the mean of all the reported values (documentary controlsFigure 6(a)). Considering the other control typologies, the situation is not different. The Treviso and Venice departments perform more management controls than the other departments while Padova performs more technical controls, achieving values that are sometimes twice as high as the mean of all the weighted values together (Figure 6(b) and 6(c)). Data for analytical controls (Figure 6(d)) appear somewhat variable regardless of the fact that some provincial departments (for example Vicenza) carry out many more analytic controls, year after year, than the total mean weighted value. In general, as disaggregated data on controls are not available with regard to plant potentiality (PE), a more accurate assessment is not possible.

Despite the annual variability, the Venice, Treviso and Vicenza provincial departments conducted an average number of analytical controls that can be compared with the other departments (Rovigo, Verona, Belluno and Padova). This can at least be partially explained if compared with the next figure (Figure 7), which depicts plant infrastructure at the regional level with reference to WWTP potentiality. In the provinces of Vicenza, Venice and Padova there is a higher number of plants with potentiality greater than 50,000 PE compared to the other provinces. These plants are subject to a higher number of analytical controls than smaller ones as established by Decree n. 152/2006 (Tables 2 and 3). On the other hand, if we consider the plants whose PE is higher than 10,000 PE, the province of Padova has the highest concentration of plants in this range (Figure 8). The differences in control data can be explained by the different priorities given to controlling pressure sources by each provincial department but above all by territorially specific characteristics that can condition control activities, leading to greater attention being given to specific pressure sources. The use of analytical verification should, logically speaking, be useful in assessing the extent to which limit values for discharges are respected. Furthermore, in collaboration with the plant manager, analytical verification supported by documentary, management and technical controls in an integrated control approach also helps to understand the causes of overruns in discharge limit values and helps for the adoption of corrective measures.

WWTP discharge quality

The WWTPs assessed in this study refer to the provinces of Venice, Treviso and Vicenza, which are important for their resident population as well as for the industrial activities present in their territories. In these provinces, there are 164 authorized WWTPs with up to 10,000 PE according to the official census (cadastre). Of these, only 127 plants have analytical control data on discharges available for the period considered (Table 8) and according to activity performed by the control Authority. On this WWTP sample the elaboration of wastewater discharge quality data has been gathered.

Samples of discharges from WWTPs, were analysed, according to Italian Decree n. 152/2006 and to the regional WPP, to determine BOD5, COD, TSS, Ntot, N-NH4, N-NO2, N-NO3, Ptot and E. coli. Based on activities present in the territory served and the characteristics of the receiving water body, other parameters like metals, surfactants (ionic, non ionic, total), hydrocarbons, halogenated organic solvents, other dangerous substances on the official test list, were defined using historical data. Values of analytical controls for plants with up to 10,000 PE in the provinces of Venice, Treviso and Vicenza in the period 2008–2015 have been recovered from the Regional Environmental Informative System (SIRAV) and produced in accordance with official sampling and analytical methods. In total, 47,917 analyses have been assessed. This assessment reveals 11,890 WWTPs <2,000 PE and 36,027 for WWTP in the 2,000–10,000 PE range. Median, min/max, 25° and 75° percentile values have been calculated. It must be remembered that for fiscal controls the single measured value is compared with the limit value and not with the mean, median or the 75° percentile value of multiple measurements, which are useful only as indicative data. Assessments have been performed for each province for the following potentiality ranges: <2,000 PE and 2,000–10,000 PE. Dangerous substances did not present values above the LOQ.

Results for the parameters BOD5, COD, TSS

The following Figure 9(a)9(c) present the situation of WWTPs in the 2,000–10,000 PE range in the provinces of Treviso, Venice and Vicenza for the period 2008–2015. They analyse BOD5, COD, TSS parameters, measured at discharge. For limit values for parameters measured at discharge in WWTPs see Table 5 (column C) as they are not reported in the figures. Limit values (adopted with the considerations and criteria already mentioned) have been sometimes exceeded except in cases where the median and the 75° percentile values were significantly lower than the limit values themselves. The situation is similar for the WWTPs in the <2,000 PE range. Despite the limitations of the data sample available, in some cases, and for certain parameters (Treviso and Vicenza), limit values have been exceeded. In general, the discharge values measured, and taken as median values, are lower than those of the limit value (Figure 9(d)9(f)).

Results for nitrogen forms and microbiology

With the exclusion of N-NO2, whose measured values have always been lower than the established limit value and have been reported only for completeness of this study, overruns of the N-NH4 and N-NO3 parameters are different for the situations registered in the provincial territories during control activities for WWTPs in the 2,000–10,000 PE range (Figure 10(a)10(c); limit values are not reported, see Table 5; Ntotal is not a regulated parameter, while it is recommended). While in the provinces of Treviso and Vicenza overruns refer to the N-NH4 parameter, in the province of Venice overruns relate to N-NO3. This last point should be investigated in depth as it relates to variations in plant management or to plants that are structured differently or that apply different processes. Nevertheless, in some cases the median appears to be lower than the limit values, even when considering the standard deviation values. This supports the fact that limit value overruns appear to be linked to particular events rather than to a plant's structural problems (i.e. overloading).

Despite the fact that there is no legal limit value for the parameter Ntot for plants with PEs below 10,000, if we take the sum of the limits for N-NH4 and N-NO3 discharges as the limit value and we compare this limit value with the sum of the values measured for the two N forms, and we further consider that together these represent the highest percentage of N-forms and that in specific situations this represents more than 80% of total N, then, in general terms, the sum of limit values is never overrun by the sum of the measured values of the two N forms. According to available data for 2008–15 the considerations for P can also be very similar; leaving aside insignificant overruns, the median values are lower than the discharge limit value.

For lower potentiality plants (<2,000 PE) median values generally tend to be lower than limit values regardless of the fact that limited data are available for N-NH4, N-NO2, N-NO3 and Ptot, (Figure 10(d)10(f)). The reasons for the registered overruns are often due to excessive hydraulic loads arriving at plants where most of the sewers are combined, rather than being attributable to ineffective plant management or structural deficiencies. The plants considered generally regard the ASP with suspended biomass, however, different structures are possible and in general for small and medium sized plants no specific unit process for N and P abatement is present.

In Figure 11(a)11(c) the E. coli parameter is reported for WWTPs <10,000 PE in the provinces of Treviso, Venice and Vicenza. It must be noted that limit values are frequently exceeded, but this is only indicative as limits are not compulsory unless required by the province itself. Prescribed E. coli values are evaluated when the receiving water body must be safeguarded for health reasons (i.e. water used for irrigation of crops, bathing water along the coast, etc.). Disinfection is imposed only along the coastline and for the larger plants.

Over the last years control Authorities in the Veneto region (North East Italy) have been paying much more attention to small (<2,000 PE) and medium (2,000–10,000 PE) sized WWTPs as regards the application of the regional WPP. The study presents the situation of small and medium sized WWTPs in this area and analyses official data available for the period 2008–15 regarding the discharge from 127 WWTPs in the provinces of Venice, Treviso and Vicenza (47,917 analyses performed by the Veneto Regional Environmental Agency), and highlights critical issues with regard to their ability to conform to discharge limit values. Organic and nutrient loads, Escherichia coli are the most critical indicators.

Despite the relatively small load processed compared to other classes of WWTPs, small and medium-sized plants (representing about the 10% of the total organic load of treatment systems in the Veneto region) are fundamental in responding to local criticalities of pollution and at the same time to favour decentralisation in areas that are not easily connectable to large centralised plants. Despite some overruns of the limit values for important parameters, both the median value and the 75° percentile considered appeared lower. Dangerous substances did not present values that were higher than the LOQ established by the analytical methodology applied. Additional investigation is required to understand the causes of the recorded overruns of the N-NH4 parameter in the provinces of Treviso and Vicenza, and of N-NO3 in the province of Venice.

Integrated controls (technical, documentary, management and analytical) appear fundamental, especially since the representativeness of analytical controls for plant functionality and reliability is incomplete. Functional verification (Ostoich et al. 2010) and the use of the Sludge Biotic Index (Madoni 1994) could be particularly useful when conducting integrated controls of small and medium sized plants; as well as reducing analytical activities it would also enhance awareness of a plant's efficiency and its ability to respect discharge limit values.

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