Clean Water Production from Urban Sewage by Algae-Based Treatment Techniques, a Reflection of Case Studies

Abstract

The inadequate collection and treatment of urban wastewater continue to pollute built environments, threaten public health, and contribute to epidemic outbreaks in many densely populated, underdeveloped regions. This study investigates whether algae-based wastewater treatment offers an optimal and efficient solution for drought-prone and underdeveloped cities. Given recent global challenges, such as the COVID-19 pandemic, nature-based wastewater treatment methods—particularly algae-based systems—have regained attention due to their feasibility, cost-effectiveness, and sustainability. Algae-based wastewater treatment presents an innovative approach to sustainable urban development, offering environmental, resource-efficient, energy-saving, and biodiversity benefits while supporting circular economy principles. This study evaluates recent advancements in wastewater treatment technologies and applies a case study methodology to Zahedan City, analyzing sewage canal networks, wastewater composition, and treatment feasibility. Three algae-based techniques were assessed, with waste stabilization ponds (WSPs) identified as the most suitable solution based on technical, economic, and environmental indicators. Key factors such as climate conditions, land-use policies, and cost-effectiveness were incorporated into the comparative analysis, enhancing the scientific rigor of this study compared to prior research. The findings provide actionable insights for urban planners, engineers, and policymakers to address simultaneous challenges in wastewater management, public health, and water scarcity.

1. Introduction

The production of clean water from urban sewage is essential for community sustainability, particularly in water-scarce cities. Since the 1980s, scholars have emphasized the necessity of water recycling as a key strategy to address water shortages, with treated wastewater now recognized as a critical resource [1,2]. From one perspective, wastewater is a major environmental problem causing stark harm to people, nature, and the environment. Wastewater poses significant environmental and public health risks, often stemming from inadequate management, technological limitations, and insufficient funding for collection and treatment systems. Many cities, including Zahedan, struggle to meet their citizens’ water demands [3]. From another view, in many cities with water scarcity, wastewater is a valuable wealth, as scholars have declared that wastewater is a valuable resource rather than a waste stream, with a focus on resource recovery. Conversely, wastewater represents a valuable resource in water-scarce regions, with growing scholarly focus on resource recovery techniques [4,5]. For instance, studies on groundwater contamination in Egypt’s Nile Delta aquifer highlight the need for expanded wastewater treatment infrastructure to enhance reuse efficiency in water-stressed, densely populated cities [6]. Treated wastewater has diverse applications, including environmental protection, public health improvement, and the provision of clean water for agriculture and drinking in arid regions. For example, water-smart management and advanced treatment methods have been proposed for Bhuj, India, to mitigate water stress and pollution amid rapid urbanization [7]. Throughout the history of urbanization, human beings have used various methods to collect wastewater, aiming to protect their health and environmental cleanliness. Later, they learned and used wastewater treatment techniques to abstract qualified water and manufacture agricultural and livestock clean production. Historically, wastewater management has evolved from basic collection methods to advanced treatment technologies, enabling safe reuse for agriculture, industry, and potable needs. Identifying suitable wastewater recycling strategies for each city is critical, whether through centralized or decentralized systems [8,9].

Recent advancements in treatment technologies—such as membrane filtration, solar–thermal processes, and capacitive deionization—demonstrate the potential of innovative solutions like algae-based systems. While some approaches rely on expensive reactors (e.g., solar-driven cogeneration), algae-based treatment offers a cost-effective alternative for freshwater production [10,11].

Wastewater treatment methods vary from simple to highly complex infrastructure, with costs differing significantly across design, construction, operation, and maintenance phases. Engineers select techniques based on local physical, climatic, and economic conditions, but all share the common goal of ensuring clean water and sustainable resource recovery [12,13].

The novelty of this research is its suggestion of algae-based wastewater treatment as a viable solution for cities with abundant natural energy and land but limited financial resources. Algae-based systems remove coliform bacteria, pollutants, nitrates, and heavy metals while reducing chemical and biochemical oxygen demand (COD/BOD).

The key research question is as follows: Can algae-based wastewater treatment serve as an optimal, efficient, and economical solution for underdeveloped cities like Zahedan? Our objective is to evaluate this technique’s applicability, considering public health risks (e.g., pathogenic viruses such as coronavirus in wastewater). The goal is to use wastewater treatment techniques in Zahedan and similar cities, considering environmental and public health issues due to dangerous viruses, such as coronavirus, in the wastewater. The goal is significant; indeed, as Rusiñol wrote, wastewater comprises a large variety of pathogenic and commensal viruses and provides important information about how they are transmitted among the population. As noted by Rusiñol [14], wastewater harbors diverse pathogens, making treatment critical for interrupting disease transmission.

This work introduces a natural wastewater treatment system leveraging local resources, including solar energy, low-cost infrastructure, minimal land use, and high efficiency in clean water production. Such systems align with waste stabilization ponds, which rely on aerobic processes to remove pathogens and generate reusable water [15,16]. The algae-based treatment technique offers five key sustainability advantages for Zahedan:

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Environmental impact: Unlike conventional methods, algae absorb nutrients and pollutants without chemical additives, reducing ecological harm.

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Resource efficiency: Algae grow using sewage-derived nutrients, recycling wastewater components and minimizing external inputs.

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Energy savings: Photosynthesis-driven treatment reduces energy demands compared to traditional systems.

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Biodiversity conservation: Algae ponds create habitats for aquatic organisms, enhancing ecosystem health.

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Circular economy: The process converts sewage into clean water and biofuels, supporting sustainable resource loops [17,18].

The following section analyzes waste stabilization ponds (WSPs) as a case study, detailing their operational mechanisms and feasibility for Zahedan.

2. Materials and Methods

2.1. Algae-Based Wastewater Treatment Techniques for Zahedan

Algae-based wastewater treatment systems—including stabilization ponds, anaerobic/facultative/maturation ponds, constructed wetlands, aerated lagoons, and oxidation ditches—rely on symbiotic relationships between oxygen-producing algae and aerobic bacteria [19]. Ho and Goethals (2020) demonstrated that modern pond systems have evolved significantly through advancements in hydraulic modeling, biogeochemical process optimization, and microbial community management [20].

Facultative pond systems, a focus of this study, employ a two-stage process:

• Primary facultative ponds receive raw wastewater.
• Secondary ponds treat effluent from anaerobic pretreatment.

Panda et al. (2021) [21] highlighted the nutrient-energy-wastewater nexus in such systems, noting their dual benefits:

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Effective removal of coliform bacteria, heavy metals, and organic pollutants (measured as BOD/COD).

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Production of valuable algal biomass for resource recovery [21,22].

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As shown in Figure 1, our proposed waste stabilization pond (WSP) system integrates

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A: Anaerobic pond;

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F: Facultative pond;

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M₁–M_n: Maturation ponds.

Figure 1. Schematic of the proposed waste stabilization pond (WSP) system for Zahedan.

Figure 1. Schematic of the proposed waste stabilization pond (WSP) system for Zahedan.

Algae-based treatment is particularly suitable for Zahedan due to the following:

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Climate compatibility: High solar irradiance (annual average: 20 °C, max 42 °C) supports year-round algal growth.

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Resource constraints: Low operational costs compared to mechanized systems [23].

2.2. Study Area: Zahedan City

Figure 2 shows the location of Zahedan City in Iran.

Zahedan (29.5°N, 60.9°E), the capital of Sistan-Baluchestan Province, Iran, faces critical water challenges. Its population is between 672,598 (official, 2023) and ~1 million (estimated). The climate of the region is BWk (Köppen classification): arid desert with <100 mm annual rainfall. Water sources in Zahedan are dependent on a 200 km pipeline from Hirmand River [24,25]. Figure 3 showws the city with different flow rates of wastewater in the streets.

Figure 3 quantifies three distinct wastewater flow regimes in Zahedan: low flow (4 × 10⁴ L/day), medium flow (3 × 10⁵ L/day), and extreme flow (2.5 × 10⁷ L/day). The figure’s color-coding system identifies urban zones by flow intensity: dark red (rapid discharge), light red (moderate discharge), and pink (slow discharge). Zahedan’s water scarcity has been a longstanding concern, with Iranian and international experts advising against urban expansion as early as the 1970s due to insufficient water resources. In 2013, authorities implemented a 200 km pipeline from the Hirmand River to alleviate water shortages [25]. The city’s subtropical BWk climate (Köppen classification) features atmospheric inversions and clear skies that inhibit rainfall, with temperatures ranging from an annual average of 20 °C to a summer maxima of 42 °C. Wastewater treatment requires precise oxygen dosing proportional to sewage quality and quantity. The stoichiometric oxygen demand for organic matter oxidation—converting pollutants to CO₂ and H₂O—is calculable. In Zahedan’s arid climate, bacterial metabolism rapidly depletes dissolved oxygen in domestic sewage. This biological process is represented by the following:

$$Wastewater+Oxygen\stackrel{Bacteria}{\to } Treated wastewater+new bacteria$$

(1)

Our proposed treatment methodology for Zahedan integrates four phases:

• Field analysis: Quantitative and qualitative characterization of wastewater sources, pathways, and discharge points.
• Material composition analysis: Identification of wastewater constituents.
• Technology selection: Evaluation of algae-based treatment options.
• Cost–benefit analysis: Comparative assessment of treatment alternatives.

2.3. Wastewater Sources and Conveyance Systems

Zahedan’s sewage network comprises two primary components:

• Industrial/Commercial Wastewater:

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  Originates from the Galambor Street desalination plant (pink zone).

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  Merges with effluent from the Mostafa Khomeini/Razmjoo Street plant (light red zone).

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  Receives additional flows from car washes, workshops, and commercial centers.

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  Characterized by elevated salinity (TDS: 1200–1800 mg/L) but lower organic loading compared to domestic sewage.

• Domestic Wastewater:

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  Generated by residential buildings, hotels, and guesthouses.

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  Contains household waste, plastics, and personal care products.

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  Flows through natural watershed channels repurposed as sewage conduits.

Four major canal systems convey Zahedan’s wastewater:

Pasdaran Canal:

• Origin: Shahid Qalanbar Street water plant (pink zone).
• Route: Military area discharge at Azadi Square → Razmjoo district (light red) → Kamposia village (north).

Azarakhshi Canal:

• Origin: Mehr Street (light red zone).
• Features: Historic riverbed with illegal construction.
• Pathway: Imam Khomeini/Molavi intersection → Saadi/Azadi streets (dark red) → Kamposia confluence.

Sistan Canal:

• Origin: Kosar Square neighborhoods (dark red).
• Characteristics: Open channel wider than Taftan/Makran streets.
• Termination: Lar River (northern outskirts).

Abazar Canal:

• Origin: Tabatabai Street (dark red).
• Historic seasonal watershed along Resalat Street.
• Joins Sistan Canal before terminating in Lar River.

Wastewater volumes increase progressively from southern (pink) to northern (dark red) zones due to cumulative household connections and seasonal flow variations.

2.4. Wastewater Composition Analysis

Zahedan’s sewage contains the following:

• Organic matter: Fecal sludge, food waste, personal care products.
• Particulates: Plastics, paper, vegetable matter (TSS: 150–400 mg/L).
• Pathogens: Enteric bacteria, viruses (including SARS-CoV-2 RNA).
• Chemical contaminants:

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  Nitrogen compounds (NH₄⁺: 25–40 mg/L).

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  Phosphates (PO₄³⁻: 10–15 mg/L).

The wastewater’s anaerobic conditions (DO < 0.5 mg/L) promote septic decomposition, generating malodorous hydrogen sulfide (H₂S).

2.5. Sustainable Treatment Solutions

While hybrid green–gray systems show promise for pathogen removal [26,27], Zahedan requires decentralized solutions due to the following:

• Prohibitive costs of centralized infrastructure (USD 120–180 per capita).
• Limited public/private investment capacity.
• Advantages of modular systems:

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  Scalability to population density.

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  Reduced land requirements (0.2–0.5 m²/person).

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  Lower energy consumption (0.1–0.3 kWh/m³) [28,29,30].

2.5.1. Wastewater Stabilization Ponds

• Process: Sequential anaerobic → photosynthetic treatment [31,32].
• Advantages:

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  99.9% pathogen removal efficiency.

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  Biomass production (0.5–1.2 kg/m³/day).

• Operational parameters:

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  HRT: 20–30 days.

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  Depth: 1.5–2.0 m [33,34].

2.5.2. Constructed Wetlands

• Mechanism:

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  Nitrification (NH₄⁺ → NO₃⁻) via phragmite roots.

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  Denitrification (NO₃⁻ → N₂) in anoxic zones [35,36].

• Design:

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  Surface flow configuration.

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  Hydraulic loading: 50–100 mm/day.

2.5.3. Facultative Ponds

• Design equation:

  $${\lambda }_s={\displaystyle \frac{10L_{i}Q}{A_f}}$$

  (2)

Here, λ_s = surface loading rate (kg BOD/ha/day), L_iQ = influent BOD mass (g/day), and $A_f$ = pond area (m²).

• Key features:

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  Algal photosynthesis maintains DO > 3 mg/L.

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  Thermocline formation at 300–500 mm depth.

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  Optimal depth: 1.0–1.8 m [37,38,39].

3. Results

Theoretical analyses and practical workshop findings demonstrate that algae-based wastewater treatment is a viable method for recovering clean water and other products in Zahedan. Compared to advanced technological reactors, which require substantial investments in construction, operation, maintenance, and repairs, nature-based systems—such as waste stabilization ponds (WSPs), constructed wetlands, and facultative ponds—prove more practical. These systems leverage Zahedan’s climatic and natural resources, including abundant land, high solar irradiance, consistent wind patterns, and elevated temperatures. To optimize urban sustainability, algae-based wastewater treatment infrastructure should be integrated into the urban planning and physical design of underdeveloped cities like Zahedan. Specifically, decentralized algae-based treatment plants should be constructed in each urban district, with their scale determined by local household density and sewage inflow volumes.

Key Advantages of Algae-Based Treatment**

• Gravity-driven hydraulic cycles reduce energy demands.
• High treatment efficiency, with up to 90% waste separation, yielding clean water from wastewater.
• Microalgae proliferation (observed as dark-green biomass in all three systems) enhances nutrient removal.

Technical Observations:

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The sewage exhibited high chlorophyll concentrations, confirming the presence of photoautotrophic algae.

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In facultative and maturation ponds, oxygen produced by algae serves as a substrate for heterotrophic bacteria.

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The BOD (biochemical oxygen demand) of facultative pond effluent was derived from Equation (3):

$$L_e={\displaystyle \frac{L_i}{1+K_1{Q}_f}}$$

(3)

where $L_i$ is the BOD of wastewater per mg/L and $K_i$ is the first-order rate for BOD deletion. The term $L_e$ is the unfiltered BOD of the algae in the facultative pond effluent. Algal biomass concentrations in facultative ponds are subjective to solar radiations, the BOD loading (${\lambda }_s$), the un-ionized ammonia absorption, and the un-ionized $H^{2}S$ application.

To design a functional anaerobic pond to treat the wastewaters coming from Zahedan’s canals, we have the following design parameters:

Algal biomass concentrations depend on solar radiation, BOD loading, concentrations of un-ionized ammonia (NH₃), and hydrogen sulfide (H₂S). For a population of 500,000 (generating 50,000 m³/day at 100 L/person/day), we have the following: influent BOD ($L_i$ = 300 mg/L), temperature (T = 25 °C), pond depth = 3 m, calculated surface area ($A_f$ = 14,300 m² (from Equation (3)), and total volume = 42,900 m³.

4. Discussions

In many developing cities—particularly those affected by conflict, drought, or underdevelopment—domestic and municipal sewage is often discharged directly onto streets. This practice not only degrades urban aesthetics but also poses significant risks to public health and environmental sustainability. To address this issue, engineers commonly advocate for low-cost wastewater collection and treatment solutions. Among these, algae-based wastewater treatment has emerged as a viable option due to its reliance on natural resources such as sunlight, wind, favorable temperatures, and the availability of unused land. Numerous countries have successfully implemented this method, enabling wastewater reuse for industrial, agricultural, and urban greening purposes.

To evaluate the economic viability of algae-based wastewater treatment, we conducted a cost–benefit analysis, comparing three systems—waste stabilization ponds, constructed wetlands, and facultative ponds—with Stockholm’s advanced Käppalaverket wastewater treatment plant. The Käppalaverket facility, one of the world’s most efficient treatment plants, serves over 500,000 residents and employs state-of-the-art mechanical, chemical, and biological technologies to treat wastewater before discharging it into natural water bodies [40].

Our analysis focused on Zahedan, an economically disadvantaged city lacking adequate infrastructure and facing challenges such as sanctions, poverty, and governance issues. In contrast to Sweden’s cold climate and limited solar exposure, Zahedan’s abundant sunlight and land availability make it well suited for algae-based systems. The economic comparison revealed that all three algae-based techniques are significantly less expensive than the high-tech Käppalaverket plant. The calculations assumed a service population of 500,000, with a wastewater flow rate of 120 L per capita per day and an oxygen injection rate of 40 g per person per day. Based on empirical observations, we estimated that each liter of influent requires 25 mg of oxygen. Additionally, the land area requirements for each system were calculated (

Table 1. Comparative costs and revenues of three algae-based wastewater treatment techniques (in thousands of USD) versus the Käppalaverket reactor.

Wastewater Treatment Technique	Käppala Plant in Stockholm	Waste Stabilization Pond in Zahedan	Constructed Wetland in Zahedan	Facultative Pond in Zahedan
Initial Investment	30,000	600	750	800
Construction cost	10,000	400	550	700
Operating and maintenance costs in 1 year	7000	50	80	120
Total costs	47,000	1050	1380	1620
Irrigation income in 1 year	3000	200	250	300
Income from fish farming in 1 year	42,000	100	125	140
Revenue from fertilizer sales in 1 year	45,000	150	150	190
Total revenue in 1 year	90,000	450	525	630
Estimated profit in 1 year	43,000	60	755	990
Land area/hectare	30	46	50	30

).

The results demonstrate that waste stabilization ponds are the most economical option, requiring the lowest initial investment while generating annual revenues of up to USD 450,000** from irrigation, aquaculture, and fertilizer sales. Although this method relies on conventional technology, its affordability aligns with Zahedan’s economic constraints. Constructed wetlands and facultative ponds, while slightly more expensive (at USD 1,380,000 and USD 1,620,000, respectively), remain far more cost-effective than the USD 47,000,000 required for the Käppalaverket plant. The latter’s advanced biofiltration technology entails high capital and operational costs, making it financially impractical for Zahedan. A further comparison of annual benefits (Figure 4) reveals that facultative ponds offer the highest profitability, potentially boosting Zahedan’s local economy.

Land-use requirements were also assessed (

Table 2. Urban land area required for each algae-based wastewater treatment technique.

Name of Technique	Käppala Plant in Stockholm	Waste Stabilization Pond in Zahedan	Constructed Wetland in Zahedan	Facultative Pond in Zahedan
Required urban land area/hectare	30	46	50	30

Source: Field studies conducted by the authors.

).

From an urban planning perspective, facultative ponds require the least land, further supporting their feasibility for Zahedan. We also evaluated the environmental performance of each system by measuring average pollutant concentrations (

Table 3. Average pollutant concentrations in effluent from the three treatment techniques.

Name of Technique	Waste Stabilization Pond		Constructed Wetland		Facultative Pond
	Influent	Effluent	Influent	Effluent	Influent	Effluent
BOD/mg	200–600	10–30	200–600	10–40	200–600	10–30
COD/mg	300–800	30–150	300–800	20–200	300–800	30–150
TSS/mg	150–400	20–60	150–400	5–30	150–400	20–60
TKN/mg	30–100	5–30	30–100	5–25	30–100	5–30
TP/mg	5–25	1–5	5–30	1–15	5–25	1–15
FCB/100 mL	${10}^4$ to ${10}^7$	$<200$	${10}^4$ to ${10}^7$	$<200$	${10}^4$ to ${10}^7$	$<200$

Source: Field laboratory observations by the authors.

). System efficiency varied depending on design, retention time, influent characteristics, and climate conditions.

In Table 3, the key parameters included are the following:

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BOD (biochemical oxygen demand);

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COD (chemical oxygen demand);

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TSS (total suspended solids);

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TKN (total Kjeldahl nitrogen);

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TP (total phosphorus);

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FCB (fecal coliform bacteria).

Environmental impact assessments indicated negligible differences in pollutant removal efficiency across the three systems. However, facultative ponds demonstrated superior outcomes in terms of environmental sustainability, operational efficiency, and long-term durability. These findings align with prior research affirming the applicability of low-cost, ecologically friendly wastewater treatment systems in resource-constrained settings [41,42].

5. Conclusions

Urban wastewater poses significant risks to both the environment and public health. To address this issue, natural energy- and algae-based techniques—such as waste stabilization ponds, constructed wetlands, and facultative ponds—have been evaluated for their potential to sustainably recover clean water. This study proposed a novel decentralized wastewater treatment system tailored to Zahedan City, offering benefits for public health, environmental protection, the circular economy, and resource efficiency. Zahedan’s climate—characterized by abundant sunshine, consistent wind power, and favorable temperatures—was factored into the economic analysis and comparative assessments.

Among the three techniques analyzed, facultative ponds emerged as the optimal natural energy-algae-based wastewater treatment solution when benchmarked against the Käppala Plant in Stockholm. The facultative pond system in Zahedan demonstrated high efficiency in wastewater purification, leveraging natural light, oxygen, and bacterial activity. Our measurements indicated significant reductions in key pollutants:

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BOD decreased from 200–600 mg/L to 10–30 mg/L;

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COD declined from 300–800 mg/L to 30–150 mg/L;

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TSS dropped from 150–400 mg/L to 20–60 mg/L;

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TKN was reduced from 30–100 mg/L to 5–30 mg/L;

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TP levels fell from 5–25 mg/100 mL to below 200 mg/100 mL in the effluent.

Economic analysis confirmed that this natural energy-algae-based approach offers a logical, environmentally sound, and sustainable method for clean water abstraction.

However, a critical gap persists between research on natural energy-algae-based wastewater treatment and real-world implementation. Bridging this gap will require increased investment in pilot and full-scale projects to advance environmental cleanliness, public health, and sustainable development globally.