Technology Classification, Application Scenarios and Future Development Trends

Abstract

With the continuous upgrading of domestic wastewater treatment technologies in China, underground and semi-underground integrated wastewater treatment facilities have been widely applied for stationary decentralized sewage. However, for temporary scenarios such as construction camps and post-disaster resettlement sites, fixed underground equipment has obvious shortcomings including large land occupation, low resource utilization rate and poor flexibility. In contrast, mobile domestic wastewater treatment devices feature rapid deployment, high resource efficiency and strong adaptability, showing broad application prospects for emergency and decentralized sewage treatment. This paper summarizes the core treatment processes of mobile integrated domestic wastewater equipment developed in recent years, and introduces the structural configuration and treatment performance of devices based on physicochemical methods, biological methods, combined technologies and constructed wetlands. A systematic comparison between mobile and underground systems is conducted from the perspectives of resource utilization and long-term planning, highlighting the flexibility and economic practicability of mobile solutions. Combined with the current status of rural domestic wastewater treatment in China, a future operation mode combining small-scale sewage treatment plants and mobile treatment devices is proposed. The development trends of mobile equipment are also prospected, with emphasis on the driving role of advanced catalytic oxidation and membrane separation technologies, providing theoretical and practical guidance for technology upgrading and application promotion.

1. Introduction

With rapid economic development and rising living standards, domestic water consumption in China’s towns and rural areas continues to grow. Statistics show that by 2021, the annual domestic water consumption of established towns in China reached 6.49 billion cubic meters, but the treatment rate of rural domestic sewage was only 61.95%. The mismatch between rural environmental construction and economic development, along with uncontrolled discharge of domestic sewage, has led to increasingly serious water pollution problems.

For centralized residential areas, underground integrated wastewater treatment equipment is widely used due to its advantages of simple installation, low operating cost and low construction investment. However, for temporary sewage sources such as construction camps, post-disaster reconstruction sites and large outdoor event venues, fixed underground equipment has significant limitations: it is difficult to move and disassemble, and will become idle after the temporary demand ends, resulting in serious resource waste. Mobile integrated wastewater treatment devices, with the advantages of small footprint, compact structure, high treatment efficiency and free mobility, are more suitable for these scenarios.

With the improvement of treatment efficiency and miniaturization of equipment, the mobility of integrated wastewater treatment devices has been greatly enhanced, and their application scenarios continue to expand. This paper systematically sorts out the process types, structural characteristics and application status of mobile integrated domestic wastewater treatment equipment, analyzes their advantages and limitations, and discusses future development directions, so as to provide reference for technology research and engineering application.

2. Core Treatment Processes for Mobile Integrated Equipment

According to working principles, core treatment processes of mobile integrated equipment can be divided into two categories: biological treatment and physicochemical treatment. Combined processes integrating multiple technologies are also widely used to improve effluent quality. The core characteristics of mainstream processes are shown in Table 1.

Table 1 Core Treatment Processes for Mobile Domestic Wastewater Equipment

2.1 Biological Treatment Processes

Biological treatment is the most widely used core process for mobile domestic wastewater equipment due to its low cost and stable performance.

2.1.1 AO/AAO Series Processes

The anoxic-oxic (AO) process is a typical activated sludge process. In the anoxic tank, denitrification reduces nitrate and nitrite to nitrogen gas, and macromolecular organics are degraded into small molecules to improve the treatment efficiency of the subsequent aeration tank. The aerobic tank completes nitrification and organic matter degradation. The anaerobic-anoxic-oxic (AAO) process adds an anaerobic tank before the anoxic zone to realize anaerobic phosphorus release of polyphosphate-accumulating organisms, enhancing biological phosphorus removal.

For rural domestic sewage treatment, studies have applied a two-stage AO biological contact oxidation process with flexible bio-rope as packing, which improves biofilm adhesion and reduces sludge production, achieving Class 1A effluent standards.

2.1.2 Biofilm Processes

In biofilm processes, microorganisms attach to the surface of fillers to form biofilms, which purify sewage through contact with pollutants. Mainstream processes include biological aerated filters and rotating biological contactors.

Biological aerated filters, developed in the 1980s, combine biological degradation and filtration functions. With high-density sedimentation tanks as pretreatment, they can achieve stable Class 1A effluent standards. Rotating biological contactors feature alternating contact between rotating discs and wastewater/air, completing an AO cycle per rotation, with good nitrogen and phosphorus removal performance for small-town domestic sewage.

2.2 Physicochemical Treatment Processes

Physicochemical processes are commonly used for mobile devices treating grey water or as advanced treatment units to improve effluent quality.

2.2.1 Membrane Separation Technology

Membrane separation achieves selective separation through pressure difference across the membrane. Common types in wastewater treatment include microfiltration, ultrafiltration, nanofiltration and reverse osmosis. With the advantages of small footprint and high effluent quality, membrane technology is widely used in mobile equipment for marine and offshore platforms, meeting IMO maritime discharge standards.

2.2.2 Electrocoagulation

Electrocoagulation usually uses iron or aluminum as anodes, which generate hydroxide flocs under alkaline conditions, removing pollutants through charge neutralization and sweep flocculation. It has particularly high phosphorus removal efficiency. When coupled with electrolysis as advanced treatment for ship domestic sewage, it can achieve a COD removal rate of up to 93% under optimal conditions.

2.2.3 Advanced Catalytic Oxidation

Advanced catalytic oxidation processes degrade pollutants by generating highly reactive hydroxyl radicals, and are effective for refractory organics and emerging contaminants such as pesticides, pharmaceuticals and personal care products. Mainstream technologies include electro-Fenton, electrocatalysis and photocatalysis, with characteristics shown in Table 2.

Table 2 Characteristics of Advanced Catalytic Oxidation Processes

2.3 Combined Treatment Technologies

Combined multi-stage processes can significantly improve treatment stability and effluent quality, which is the mainstream development direction of mobile integrated equipment. The most typical combined process is the membrane bioreactor (MBR), which integrates membrane separation and biological treatment. By replacing the secondary sedimentation tank with membrane modules, it maintains high activated sludge concentration, reduces floor space and improves effluent quality.

AO-MBR combined processes can achieve better nitrogen and phosphorus removal than conventional sequencing batch activated sludge processes, with smaller land occupation. Coagulation-AO-MBR combined processes can further achieve Class 1A effluent standards. Bio-ecological combined systems, coupling biological treatment with constructed wetlands for advanced nitrogen and phosphorus removal, also have good application prospects.

3. Classification of Mobile Integrated Domestic Wastewater Equipment

Based on different core processes, mobile integrated domestic wastewater treatment equipment can be divided into four categories: physicochemical process-based, biological process-based, combined technology-based, and constructed wetland-based systems.

3.1 Physicochemical Process-Based Equipment

Physicochemical process-based equipment is mainly used for grey water treatment (washing, bathing and kitchen wastewater) rather than high-strength black water. After coagulation, filtration and disinfection, the effluent can be reused. Typical products include mobile sewage filtration vehicles, which integrate stirring, pumping and filtration units, realizing on-site grey water recycling.

For black water treatment, microfiltration and ultrafiltration are commonly adopted. Mobile fecal treatment devices integrating adsorption, microfiltration and ultrafiltration can achieve 81% COD removal, 80% SS removal and 98.4% total coliform removal, meeting local discharge standards. Mobile photocatalytic devices, which can use sunlight during the day and built-in light sources at night, have low energy consumption and strong shock load resistance.

3.2 Biological Process-Based Equipment

Biological process-based equipment is the most widely used type due to its low cost and good treatment effect. The typical process configuration includes grid, equalization tank, core biological reaction tank and disinfection tank.

AO-process mobile devices are mostly used for rural domestic sewage, with fully automatic operation control. AAO-process mobile devices adopt multi-stage segmented contact oxidation in the aerobic tank, with gradually decreasing reaction load and strong shock load resistance. Mobile treatment vehicles based on AAO process integrate filter pressing, anaerobic, anoxic and aerobic units, with ultraviolet disinfection at the effluent end, suitable for small-scale temporary sewage treatment.

3.3 Combined Technology-Based Equipment

Combined technology devices are adopted to cope with large fluctuations in water quality and quantity and maintain stable effluent quality. AO-MBR integrated mobile emergency treatment devices can achieve Class 1B effluent standards suitable for greening and irrigation. Unit-type membrane biological mobile stations integrate biological treatment and hollow fiber membrane filtration, with compact structure and no excess sludge discharge.

Some devices further combine biological and physicochemical processes, such as units integrating pulverization, sand filtration, activated carbon adsorption and photovoltaic power supply. With photosynthetic bacteria in the anaerobic chamber, they can operate independently in off-grid areas, suitable for remote field scenarios.

3.4 Mobile Constructed Wetland Systems

Constructed wetlands are rarely used in mobile devices due to large footprint and limited treatment capacity. However, they have the advantages of triple degradation mechanisms (substrate, plants, microorganisms), low energy consumption and excellent nitrogen and phosphorus removal performance.

For temporary large-scale events such as music festivals, mobile constructed wetland systems mounted on trailers have been developed. Coupled with ultrafiltration and reverse osmosis, they can achieve 90% COD removal, 95% BOD removal and 97% SS removal, producing reusable water. Reed root-based mobile treatment devices, with pretreatment units, can handle 1200 L/d of temporary domestic sewage from construction sites and camping sites.

4. Performance Characteristics and Application Status

4.1 Core Advantages of Mobile Systems

Compared with fixed underground treatment equipment, mobile integrated devices have irreplaceable advantages for decentralized and temporary sewage scenarios:

1. Saving pipeline network investment: For remote scattered areas with few residents and long distances from municipal sewage plants, laying sewage pipelines is economically unfeasible. Mobile equipment can be deployed on demand with flexible process configuration, avoiding large-scale infrastructure investment.
2. High resource utilization efficiency: For temporary construction camps and disaster relief sites, fixed treatment facilities will become idle after the demand ends, resulting in waste of capital and land resources. Mobile devices can be transferred to other sites after use, significantly improving resource utilization and conforming to the concept of sustainable development.
3. Fast deployment and emergency response: For post-disaster scenarios where the pipe network system is damaged, mobile equipment can be quickly deployed to provide temporary sewage treatment capacity, ensuring environmental hygiene and resident health.

4.2 Limitations and Existing Challenges

Mobile equipment also has inherent limitations:

• Limited treatment capacity: Due to size and mobility constraints, the treatment scale of single mobile equipment is small, unsuitable for large-volume sewage.
• Relatively high operation and maintenance cost: Membrane-based mobile devices require regular cleaning and maintenance, and coagulant and other chemical agents are often needed, leading to higher O&M costs than large fixed sewage plants.

At present, the popularization of mobile equipment in China’s rural areas also faces three major constraints: insufficient local financial support, lack of unified technical standards and discharge standards, and shortage of professional operation and maintenance personnel.

4.3 Application Status in China’s Township Areas

Statistics show that the treatment capacity of integrated wastewater treatment devices in China’s established towns has reached 23.6184 million m³/d by 2021, close to the capacity of formal sewage treatment plants, indicating that integrated treatment devices have been widely popularized in township areas. Mobile integrated devices, as an important supplement, play an increasingly important role in scattered and emergency sewage treatment.

Economic analysis shows that the treatment cost of AO-process integrated rural sewage treatment equipment is about 1.5–2.0 yuan/m³, higher than the ~1 yuan/m³ cost of large urban sewage plants. Therefore, a differentiated treatment mode is recommended: build centralized sewage plants for densely populated areas, and adopt “on-demand appointment” mobile integrated equipment for scattered small-flow areas.

5. Engineering Application Insights

5.1 Applicable Scenarios

Mobile integrated domestic wastewater treatment equipment is most suitable for the following scenarios:

• Temporary construction camps, mining sites and field engineering projects
• Post-disaster resettlement sites and emergency environmental rescue
• Large outdoor events, music festivals and tourism camping sites
• Scattered rural settlements where pipeline laying is uneconomical
• Temporary upgrading and renovation of existing sewage treatment plants as standby treatment capacity

5.2 Process Selection Guidelines

• For grey water recycling scenarios with low pollutant concentration, coagulation-filtration-disinfection processes are preferred, with low cost and simple operation.
• For conventional domestic sewage with discharge standards above Class 1B, AO/AAO biological processes are recommended, with good cost performance.
• For scenarios requiring high effluent quality or reuse, MBR combined processes are the optimal choice, with stable effluent and small footprint.
• For scenic spots and parks with landscape requirements, mobile constructed wetland combined systems can be selected, taking into account treatment and landscape functions.
• For high-strength or refractory sewage, advanced oxidation processes can be added as advanced treatment units.

5.3 Operation and Maintenance Best Practices

• Establish regular inspection mechanisms for core components such as aeration systems, membrane modules and dosing devices to detect faults in time.
• For membrane-based equipment, formulate regular chemical cleaning plans according to operating conditions to slow down membrane fouling and extend membrane service life.
• Promote intelligent remote operation systems to reduce the demand for on-site professional personnel, especially suitable for remote rural and field scenarios.
• Formulate targeted start-up and shutdown operation procedures for intermittent operation scenarios to ensure rapid recovery of biological system activity after reactivation.

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we believe that mobile integrated wastewater treatment systems are a core solution for decentralized and temporary sewage treatment, perfectly matching the growing demand for flexible, fast-deploying environmental infrastructure in China’s rural revitalization and emergency management.

Our skid-mounted mobile integrated treatment systems adopt standardized modular design, integrating AO/MBR core processes, automatic dosing systems and intelligent control units, which can be quickly deployed on truck-mounted or container platforms. The equipment is factory prefabricated and debugged, realizing plug-and-play on site, greatly shortening the construction period. Equipped with our cloud-based remote operation and maintenance platform, the system realizes real-time monitoring of water quality and equipment status, automatic fault alarm and parameter optimization, reducing the demand for on-site professional operators by more than 70%, which effectively solves the pain point of insufficient professional management personnel in rural and remote areas. We also provide customized process packages for different scenarios such as rural scattered sewage, construction camps and emergency rescue, helping clients achieve stable standard discharge at the lowest life-cycle cost.

7. Conclusion and Outlook

Mobile integrated domestic wastewater treatment equipment is an important supplement to China’s sewage treatment system, which improves the completeness of the treatment system and has high application value for temporary and decentralized sewage scenarios.

1. Core processes of mobile equipment include biological methods, physicochemical methods and combined technologies. AO-series biological processes are currently the mainstream core technology due to their high cost performance, while membrane separation and advanced oxidation technologies will further expand the application boundary of mobile equipment.
2. Compared with fixed underground equipment, mobile devices have outstanding advantages in flexibility, resource utilization and deployment speed for temporary and scattered sewage treatment, but have limitations in treatment scale and operation cost.
3. For China’s rural domestic sewage treatment, a combined mode of centralized small sewage treatment plants and mobile integrated equipment should be adopted to achieve the optimal balance of treatment effect and economic cost according to the distribution characteristics of settlements.
4. In the future, with the development of anti-fouling membrane materials, low-cost advanced oxidation technologies and intelligent operation systems, mobile integrated wastewater treatment equipment will have better treatment performance, lower operation cost and wider application scenarios, playing a more important role in China’s water environment governance.

FAQ

Q1: What are the main differences between mobile integrated and underground fixed wastewater treatment equipment?
Underground fixed equipment is suitable for long-term stable sewage sources, with low unit treatment cost but high initial investment and no mobility. Mobile integrated equipment can be flexibly transferred and deployed on demand, avoiding idle waste of resources, and is more suitable for temporary and scattered sewage sources, with relatively higher unit treatment cost.

Q2: What scenarios are most suitable for mobile domestic wastewater treatment systems?
Mobile systems are most cost-effective for temporary scenarios including construction camps, post-disaster resettlement sites and large outdoor events, as well as for scattered rural settlements where laying sewage pipelines is uneconomical. They can also be used as temporary standby capacity during the maintenance and upgrading of existing sewage plants.

Q3: What are the common operation and maintenance challenges of mobile wastewater equipment?
The main challenges include: membrane fouling for MBR-type equipment requiring regular cleaning; intermittent operation leading to unstable activity of biological systems; and lack of professional operation personnel in remote application areas. Intelligent remote operation systems and standardized maintenance processes can effectively solve these problems.

Q4: What is the core development direction of mobile integrated wastewater treatment technology?
The core development directions include: high-efficiency anti-fouling membrane materials to reduce the maintenance cost of membrane processes; low-cost advanced oxidation technologies to improve effluent quality for refractory sewage; and intelligent and unmanned operation systems to adapt to decentralized application scenarios with insufficient professional management personnel.