Sustainable Municipal Sewerage System Solution: A Case Study of Ropice ^†

Abstract

This article deals with the design of a technically, legislatively, and economically balanced sewerage system for the municipality of Ropice, which has long lacked a central sanitary sewer system. On the basis of the analysis of the territorial conditions, hydro-technical calculations, and legislative requirements, two potential solutions are compared—a decentralized domestic WWTP and a central separate sewerage system with a treatment plant. The final concept favors the central solution in order to ensure operational reliability, sustainability, and the possibility of using grants. This study provides a model example of an applicable solution for rural settlements with similar wastewater management problems.

1. Introduction

Systematic and legally compliant wastewater management is a key element in the protection of the environment and public health and the promotion of sustainable municipal development. In the Czech Republic, this area is regulated primarily by Act No. 254/2001 Coll., on Water [1], Act No. 274/2001 Coll., on Water Supply and Sewerage [2], and related implemented regulations. These legal standards define the obligation of municipalities to ensure that municipal wastewater is discharged and treated in a way that does not cause pollution of surface or groundwater while, at the same time, allowing for the control of discharged substances and compliance with emission limits in accordance with the Government Regulation No. 401/2015 Coll. [3].

The issue of sewage disposal is particularly serious in small settlements and rural areas with scattered buildings, where septic tanks without outlets, obsolete septic tanks, or inadequate domestic sewage treatment plants are still often used. Such systems are often difficult to control; they have fluctuating efficiency and pose a risk of pollution to the water environment. The long-term discharge of inadequately treated or raw wastewater into the surrounding environment leads to the eutrophication of watercourses and the contamination of groundwater with nitrates, fecal bacteria, and antibiotic residues, which contribute to the degradation of the ecological stability of the landscape.

From the perspective of the European Union, proper management of urban wastewater is addressed in the Water Framework Directive 2000/60/EC [4] and Directive 91/271/EEC on urban wastewater treatment [5], and the Czech Republic is obliged to transpose and comply with these requirements. The provision of central sewerage systems with an adequate level of treatment is also a condition for obtaining financial support from EU funds, in particular from Operational Program Environment (OPE).

The presented study focuses on the municipality of Ropice (Frýdek-Místek District), which has long been struggling with the absence of a central sewerage system. The intention is to propose an optimal technical solution for the removal and treatment of sewage water that will be

• in compliance with current legislation and technical standards (e.g., ČSN 75 6101 [6]);
• economically feasible for the municipal budget;
• operationally sustainable with minimum maintenance requirements;
• beneficial in terms of environmental protection and quality of life of the inhabitants.

The chosen approach combines research into existing legislation, analysis of the territorial and technical conditions, and a hydro-technical design of a separate sewage system including a central wastewater treatment plant. The result is a conceptual solution that takes into account not only the technical possibilities and geometry of the area, but also the long-term operational sustainability and financial viability for the municipal government.

2. Characteristics of the Area of Interest

The municipality of Ropice is located in the south-eastern part of the Frýdek-Místek District in the Moravian-Silesian Region, in the natural catchment area of the town of Třinec. The cadastral territory of the municipality covers an area of approximately 10.11 km². As of 1 January 2023, there were 1752 inhabitants, and in recent years, the village has shown stable demographic development with a slight increase. In terms of the character of the built-up area, it predominantly consists of dispersed rural buildings, primarily family houses, with a high proportion of individual construction.

From a geomorphologic point of view, the territory is classified as part of the Podbeskydská upland area, a sub-system of the West Beskydy foothills, specifically in the Třinecká brázda unit [7]. The territory has a slightly wavy relief with an altitude of 290–400 m above sea level. The slope of the terrain usually reaches 2–6%, and in some places reaches up to 10%. These morphological conditions are favorable for gravity sewer networks.

Geologically, the municipality lies in an area made up of claystones, siltstones, and sandstones of the Rača unit of the Magura flysch, overlain by Quaternary sediments (clays, sandy loams, and gravels) [8]. Due to these conditions, the area is not included in protected areas of natural groundwater accumulation or in the zone of hygienic protection of water sources, which creates favorable conditions for the construction of sewerage infrastructure and the discharge of treated water into the surface stream.

The hydrological recipient is the Ropičanka watercourse, a left-side tributary of the Olše River. The stream flows through the central part of the municipality and represents a suitable place for the discharge of treated sewage from the proposed treatment plant. The average flow (Q₃₅₅) is in the range of 0.05–0.10 m³/s [9], which is hydro-technically sufficient to dilute the normal operational flow from the WWTP.

From a climate point of view, the site is located in the moderately warm MT7 region (according to the Quitt classification), with an average annual rainfall between 800 and 1000 mm and an average annual temperature of about 8 °C [9]. Snow cover occurs approximately 70–90 days per year. These conditions are of particular importance when designing the minimum depth of pipe installation in order to protect it against freezing.

In terms of land-use planning, the municipality of Ropice is classified as a settlement with development potential. The current land-use plan, approved in 2015 and updated in 2022, assumes the development of residential buildings mainly in the southern and south-eastern parts of the municipality. This necessitates the extension of the technical infrastructure, especially the sewerage network, which currently covers only a small part of the territory. There is still no central sewerage system in the municipality, with the exception of a local sewage treatment plant in Baliny.

The natural zoning of the area, the hydro-geological conditions, and the concentration of buildings create the preconditions for the effective design of a separate sewerage system with a directional connection to the central wastewater treatment plant.

3. Existing State of the Sewerage System

The municipality of Ropice does not yet have a central sewerage system. Most of the houses are connected to individual wastewater disposal systems, which are no longer suitable in terms of treatment efficiency, hygiene, and legislative requirements. The only exception is the area of Baliny, where a small mechanical–biological wastewater treatment plant (WWTP) of the PESL 25 type was built, to which several buildings are connected via a simple gravity sewer network. This WWTP has only a limited capacity and does not allow connection to other parts of the municipality.

In the central part of the municipality, there is a small wastewater treatment plant, of the type 20 Paod, produced by Gonap spol. s. r.o., with a capacity of 20 equivalent inhabitants (EIs), which serves several public buildings (e.g., school and local authority). In other locations, wastewater disposal is handled by means of septic tanks or drywells without subsequent treatment. In the absence of an effective control mechanism, it cannot be demonstrated with certainty that all contents of these septic tanks are regularly drained and properly disposed of. In practice, this often results in the discharge of wastewater into infiltration system facilities or watercourses, in violation of Section 38 of the Water Act [8].

The use of non-functioning septic tanks with overflows into storm drains or directly into watercourses can serve as a typical example. Such solutions are not able to ensure sufficient reductions in nitrogen and organic matter concentrations or water hygiene. These sources of pollution result in a significant load of fecal pollution and eutrophication of surface watercourses, especially of the Ropičanka Stream.

The lack of a unified system for the disposal of wastewater has operational and economic disadvantages, in addition to the environmental impacts. Individual systems are maintenance-intensive, the cost of draining septic tanks is high for residents, and their efficiency is low. In addition, the municipality as an administrative unit loses the opportunity to centrally control the quality of water treatment and ensure compliance with legislative obligations.

The facts presented above show that the state of the sewerage system in the municipality of Ropice is unsatisfactory and unsustainable from the legislative, hygienic, and environmental point of view. Moreover, the current solution is incompatible with grant programs (e.g., OPE), which require a central infrastructure and the possibility of controlling the output parameters of treated water.

4. Potential Solutions

Based on the analysis of the area, legislative requirements, hydro-technical calculations, and the expected development of the municipality of Ropice, two basic concepts regarding sewage water disposal and treatment were considered. Both options were evaluated in terms of their technical feasibility, investment intensity, operating costs, the possibility of obtaining grants, and compliance with environmental objectives. The comparison was carried out in the form of a multi-criterion assessment.

4.1. Option A—Individual Systems

This option assumes the use of domestic wastewater treatment plants (DWWTPs), septic tanks with after-treatment, or root treatment plants for each property separately. This system would be based on a decentralized principle, without a central sewerage network.

• Advantages:

• Lower initial investment in infrastructure.
• Possibility for gradual implementation according to the financial means of the residents.
• Limited excavation work and interference with public space.

• Disadvantages:

• Low level of control over the quality of the discharged water.
• Risk of malfunctioning and neglect of maintenance.
• Higher operating costs for individual owners.
• No direct grant support from OPE (central infrastructure required).
• Legally problematic to discharge treated water on own land.

Experience from other municipalities shows that a decentralized DWWTP system is unsustainable in the long term in locations where the density of built-up area allows for the construction of a sewerage network. There is also a lack of tools to enforce regular servicing of the facilities.

4.2. Option B—Central Separate Sewerage System

The proposed technical solution takes into consideration the construction of a separate gravity-pressure sewerage system connecting all properties to a central mechanical–biological wastewater treatment plant (WWTP) located near the Ropičanka Stream. This system combines gravity sewers made from smooth polypropylene (DN 250–300) and three pressure discharges with pumping stations (PS1–PS3).

• Main parts of the system:

• Gravity collection sewers A, B, C, and D with a total length of over 5.6 km.
• Construction of three pumping stations with discharge lines PE DN 80–100.
• One hundred and twenty-five house connections ending with inspection manholes.
• Central WWTP sized for 2000 EIs with nitrification, denitrification, and sludge separation.
• Outlet structure flowing into the Ropičanka Stream with controlled outflow.

• Advantages:

• The system fully complies with the requirements of legislation and standards (ČSN 75 6101 [6]).
• Possibility for central control and measurement of treatment efficiency.
• Simplified approval process and higher chances of obtaining grants.
• Low operating costs per capita.
• High protection of groundwater and surface water.

• Disadvantages:

• High initial investment costs.
• Need for complex project preparation and implementation in several stages.
• Dependence on energy for operation (pumping stations and WWTP).

Table 1. Comparison of the proposed wastewater treatment options.

Criterion	Option A—DWWTP	Option B—Central Sewerage System
Investment costs	Low (individual)	High (350 mil. CZK)
Operational control	Low	High (professional operator)
Outflow quality	Fluctuating	Stable, measured
Potential grant	Limited	High (OPE, SEF)
Legislation	Risky	Fully compliant
Sustainability	Low	High

provides an comparative analysis of the individual proposed options. In terms of long-term efficiency, operational safety, and compliance with environmental objectives, Option B was assessed as the most appropriate solution for the municipality of Ropice.

5. R5. Elaboration of Option B—Central Separate Sewerage System

Based on the results of the multi-criterion evaluation, option B was selected as the optimal solution for the municipality of Ropice, consisting of the construction of a separate sewerage network ending with a central wastewater treatment plant. The designed system combines gravity and pressure parts and is designed for 1500 equivalent inhabitants (EIs). The total length of the sewer network is 5682 m and also includes three pumping stations with discharge lines. The design takes into account the morphology of the terrain, existing and future development, and the legislative requirements.

5.1. Gravity Sewer Network

The gravity part of the system consists of the main sewer A and secondary sewers A1–A9, supplemented by the main sewers B, C, and D and the secondary sewers B1, C1–C9 and D1–D2. The material used is a class SN10 polypropylene pipe with the dimensions DN 250 and DN 300. The pipelines will be laid along roads with a minimum cover of 1.8 m, with some sections reaching a depth of up to 5.2 m.

Table 2. Basic parameters of the main gravity sewer A and the main sewers B, C, and D.

Sewer	DN [mm]	Length [m]	No. of Manholes	Min. Slope [%]	Max. Slope [%]
A	300	2935	67	1,1	5.8
B	250	347	11	0.8	5.8
C	250	1809	47	0.6	5.8
D	250	364	9	0.8	4.2

presents the basic parameters of the main gravity sewer A and the main sewers B, C, and D. Detailed hydro-technical calculations of the designed sanitary sewer are available from the authors of this article.

The sewers are designed with an emphasis on achieving a minimum self-cleaning velocity ≥ 0.7 m/s according to ČSN 75 6101 [6]. In areas of significant slope, drainage shafts are included to limit hydraulic surges and the erosive effects of the flow.

5.2. Pumping Stations and Discharges

In areas with an unsuitable elevation profile, three pumping stations are designed: CS1—Rakovec; CS2—Oblázky; and CS3—Zimník. Each pumping station is equipped with a pair of pumps with automatic operation, a dry septic tank, and a failure alarm. In

Table 3. Basic parameters of discharge pumping stations.

PS	Discharge	DN [mm]	Length [m]	Height Differen. [m]	Designed Perform. [L/s]	Pump Type
1	V1	80 PE	97	10.8	4.8	SIGMA GFZU
2	V2	100 PE	142	9.43	19.0	SIGMA GFZF 80/400
3	V3	80 PE	69	3.6	2.4	ASIO AS-PUMP DUO

, the basic parameters of the pumping stations for the pressure pipelines are presented.

The discharge pipes will be made from high-strength PE pipe with internal protection against abrasive wear and will mainly be routed in protective sleeves.

5.3. House Connections

A total of 125 houses will be connected to the main sewers via DN 150 pipes, ending with an inspection manhole at the property boundary. These connections will be made through a 45° branch from the main line. The length of each connection will be around 10 m on average.

Each connection will be inspectable, accessible from a public space, and marked in the register. The material used will be PP DN 150 SN10, with a minimum cover of 1.0 m.

5.4. Sewerage Manholes

A total of 167 sewerage manholes are designed to be included in the sewerage network, among which 11 are drainage manholes (made from concrete rings (DN 1000)) and the rest are continuous manholes or manholes allowing changes in direction. The covers will be made from D400 grade cast iron, with the frame set in a concrete bed. All manholes will be fitted with stainless steel steps and sealed against groundwater ingress.

5.5. Construction Design and Excavations

The width of the excavation pits is designed according to the diameter of the pipeline and the depth of laying, within the range of 1100–1200 mm. At depths of more than 3.5 m, a box shoring system is considered. The bedding will consist of a 100 mm thick layer of compacted sand, and a gravel sand fraction of 4/8 will be used for backfilling.

At crossings with roads, utilities, and streams, the pipeline will be laid in HDPE protective sleeves according to ČSN 73 6005 [10].

5.6. Wastewater Treatment Plant

The design of the WWTP was based on the hydro-technical calculation of the sewage system. The specific water demand was assumed to be 120 l/EI/day. A mechanical–biological wastewater treatment plant (WWTP) of the CAS (Conventional Activated Sludge) type is proposed, with nitrification, denitrification, sedimentation, and disposal of excess sludge. The WWTP is designed for a capacity of 1500 EIs.

The quality of the discharged water will be regularly monitored according to the requirements of Government Regulation No. 401/2015 Coll. [3].

The wastewater treatment process produces sewage sludge as a by-product which must be managed. The basic methods of management include material utilization (agriculture, reclamation, etc.) and energy utilization (anaerobic stabilization and thermal processes—incineration and pyrolysis). Experts at the Department of Environmental Engineering, Faculty of Mining and Geology, deal with the problem of treatment of sewage sludge using a pyrolysis process [11,12].

5.7. Estimation of the Economic Costs and Investment Intensity of the Solution

The economic evaluation of the proposed solution is an essential part of this technical study and serves as a basis for the municipality’s decision-making on the implementation and possible use of grants. The costs were determined on the basis of a detailed itemized budget, the catalog prices in 2023, and recommended values from the methodological material of the Institute of Territorial Development: Average Prices of Transport and Technical Infrastructure [13].

The budget includes both the construction of the sewerage network, connections, pumping stations, and WWTP and the contingency reserve (5%), as well as the costs for project documentation and engineering. In

Table 4. Investment costs according to the individual items.

Item	Costs [CZK Excl. VAT]
Construction of gravity sewerage system (incl. manholes)	268,850,677
House sewerage connections and inspection manholes	31,340,976
Pumping stations (3 pcs)	3,800,000
Discgarges from PE pipes (DN 80–100) (estimated 2500,- CZK/lm)	1,000,000
Central WWTP including the technology	29,411,226
Project preparation, technical supervision of the investor, ingeneering work (app. 2% of total investment cost)	7,000,000
Contingency reserve (5%)	16,670,144
Total (excl. VAT)	358,073,023

, the following are presented the investment costs for the construction of the sewerage system, including the wastewater treatment plant.

The number of 1455 EIs (based on the current situation and the outlook according to the land-use plan) was used for the conversion per equivalent inhabitant (EI). The resulting cost per EI is therefore approximately CZK 246,000/EI.

These costs correspond to the price level of construction contracts in 2023 and reflect the typical unit price of gravity sewerage systems including the connections (approx. CZK 6500–7500/m). The cost of the central WWTP is within the usual level for biological activation systems in this capacity category.

This economic efficiency is further supported by the fact that the project is prepared in such a way to meet the criteria for co-financing from the Operational Program Environment (OPE) or the State Environmental Fund of the Czech Republic, where the amount of the grant can cover up to 70–90% of the eligible costs.

6. Conclusions

The proposed central separate sewerage system with a mechanical–biological wastewater treatment plant represents a long-term sustainable and legally compliant solution for the municipality of Ropice. The chosen option B allows for the efficient disposal and treatment of municipal wastewater, which significantly reduces the risk of surface and groundwater pollution and improves the hygiene situation in the area.

The concept of the sewerage system has been designed with an emphasis on operational safety, controllability of the quality of discharge, and compatibility with the grant requirements of the OPE and SEF programs. The solution includes gravity sewers combined with pressure discharges, three pumping stations, house connections with inspection manholes, and a central wastewater treatment plant with a capacity of 1500 EIs. The design takes into account both the existing built-up area and the development areas according to the land-use plan.

From the environmental point of view, the project contributes to the fulfillment of the requirements of the Water Framework Directive 2000/60/EC and Directive 91/271/EEC on urban wastewater treatment, as well as national legislation (Act No. 254/2001 Coll., No. 274/2001 Coll.). The direct consequences of the implementation will include a reduction in the discharge of inadequately treated water into the countryside, an increase in the protection of the recipient (the Ropičanka Stream), and an improvement in the quality of life of the inhabitants of the municipality.

From an economic point of view, this system is an investment with a higher input requirement, but thanks to its central nature, it allows for efficient management, lower long-term operating costs, and the possibility of obtaining up to a 90% grant from public sources. The cost per equivalent inhabitant has been set at approximately CZK 246,000, which is in line with similar projects in the Czech Republic.

Overall, the proposed sewerage system is technically feasible, economically justifiable, and environmentally beneficial. Its implementation should be a strategic priority for the municipality as it will contribute significantly to the objectives of sustainable development, water protection, and the improvement of the quality of life of the inhabitants.

In terms of long-term operation, it is also necessary to consider potential operational risks that may affect the sustainability of the proposed central sewerage system. The most significant include the energy demand of pumping stations and the wastewater treatment plant, as well as the management of sewage sludge generated during the treatment process. The energy consumption can be mitigated by using energy-efficient pumps equipped with variable-frequency drives, optimizing the operation of aeration systems, and ensuring regular maintenance to prevent losses in efficiency.

Regarding sludge management, the use of advanced stabilization and dewatering methods, together with opportunities for material or energy recovery (e.g., composting, anaerobic digestion, or pyrolysis), can significantly reduce disposal costs and environmental impacts. By applying these mitigation measures, the designed system can maintain both operational reliability and economic efficiency throughout its life cycle, while fulfilling all environmental protection and sustainability requirements.