Technology for Septage Treatment ^†

Abstract

Septic wastewater, or septage, represents a specific type of wastewater with a high concentration of organic matter and significant variability in composition, which places increased demand on its treatment. With the increasing pressure for decentralized solutions for small areas with no established sewage infrastructure, technologies that can ensure stable operation of the treatment plant are coming to the fore. This paper compares the technologies used for septic wastewater treatment, i.e., sequencing batch reactor (SBR), membrane bioreactor (MBR), and aerobic granular sludge reactor (AGS). For the AGS technology, a trial run of a selected wastewater collection plant is carried out.

1. Introduction

Septage treatment is a critical component of modern sanitation approaches, particularly in rural and decentralized areas where centralized sewer systems are impractical or unavailable. Septage, characterized by high organic content, pathogens, and nutrients, poses significant environmental and public health risks if left untreated. Conventional treatment systems often fail to meet the high standard requirements of effluents, especially concerning the elimination of phosphorus and nitrogen. These shortcomings have driven the advancement of more sophisticated treatment solutions designed to improve efficiency, reliability, and long-term sustainability.

Among biological treatment technologies for septage management, three prominent process options are Sequencing Batch Reactors (SBR), Membrane Bioreactors (MBR), and Aerobic Granular Sludge (AGS). Each of these systems offers unique advantages and challenges, making them suitable for specific applications depending on factors such as wastewater composition, flow rates, energy availability, and regulatory requirements.

SBRs are renowned for their operational flexibility and compact design, performing biological treatment in a single tank through sequential phases. Aerobic Granular Sludge (AGS) technology, often implemented in SBRs, further enhances treatment efficiency by leveraging dense microbial aggregates for simultaneous carbon, nitrogen, and phosphorus removal. MBRs, on the other hand, combine biological degradation with membrane filtration to produce high-quality effluent suitable for reuse, albeit with higher energy demands and fouling concerns.

This article provides a short analysis of septage characteristics based on seven samples collected from septic tanks and cesspools with varying accumulation times. In addition to characterizing the variability of septage, the article also provides a review of those three treatment technologies. Furthermore, the article reports on a trial run of AGS technology, evaluating its performance under septage conditions.

2. Septage

Septage is the combination of liquids and sludge removed from on-site sanitation systems (such as septic tanks), consisting of untreated and partially treated sewage solids of domestic origin [1]. In practice, septage tends to be a highly concentrated and variable waste that often contains significant grease, grit, hair, and other waste, and it is characterized by a foul odor and poor dewaterability [2] (p. 4). These properties make septage difficult to handle and treat, and untreated septage can harbor disease-causing pathogens (viruses, bacteria, and parasites) [2] (p. 4).

In this study, seven septage samples were collected from different septic tanks and cesspools, representing a range of accumulation times. Physiochemical analyses included parameters such as BOD₅, COD, total nitrogen (TN), total phosphorus (TP), and pH, etc. The samples were always homogenized before collection from fecal trucks.

Table 1. Characteristics of septage [3].

Concentration (mg/L)
Parameters	Minimum	Maximum	Indicative Values
pH	6.42	9.6	6–8.5 [4]
Ammoniacal nitrogen	0.8	124.4	20–45 [4]
Total nitrogen	175	1239.6	30–70 [4]
Total phosphorus	10.2	120	5–15 [4]
Chemical oxygen demand	754	13,970	250–800 [4]
Biochemical oxygen demand	289	3000	150–400 [4]
Total solids after drying at 105 °C	1339	35,368	---
Total solids after drying at 550 °C	769	5018	---

lists the characteristics of analyzed septage.

Table 2. Sources of septage [3].

Object	Storage Time	Type of Wastewater
Septic tank	6 months	municipal
Cesspool	3 weeks	from the sports hall
Septic tank	3 years	municipal
Cesspool	3.5 days	municipal + industrial
Septic tank	14 days	municipal
Septic tank	6 weeks	sales warehouse, 4 people
Septic tank	4 weeks	sales warehouse, 4 people

then explains from which object the samples were collected, after what period of time, and the character of the wastewater.

As shown in Table 1, the characteristics of septage exhibit considerable variability. The acceptable pH values for effluent generally fall between 6 and 8.5. While the lower limit corresponds to typical wastewater conditions, the observed upper value of 9.6 indicates that certain septage samples were alkaline.

Total nitrogen concentrations were extremely elevated, indicating significant nitrogen loads, most probably due to concentrated urine and fecal matter in the septic system. Similarly, total phosphorus levels exceeded the expected range. Elevated phosphorus is commonly associated with fecal solids, residues from detergents, and biochemical reactions within the tank.

In terms of BOD, the lower limit is still acceptable; the upper limit of 3000 mg/L is very high in biodegradable organic matter. While analyzing the samples, one sample had such a high concentration of BOD that it was not possible to determine the BOD value. A large dilution of the sample would have been required, which would have caused an exponential error in the determination. The assumed value was above 6000 mg/L.

3. Overview of Technologies

This paper presents a review of treatment technologies that could be used for septage treatment, focusing on three key systems: Sequencing Batch Reactor (SBR), Membrane Bioreactor (MBR), and Aerobic Granular Sludge (AGS).

• SBRs is a fill-and-draw activated sludge system for wastewater treatment. They are particularly effective for intermittent and low flows. The operation follows a series of phases: fill, react, settle, draw, and idle phases [5].
• MBRs combine biological treatment and membrane filtration to produce high-quality reusable effluent [6]. Though they achieve better removal of pollutants, they are plagued by problems like membrane fouling, energy requirements, and maintenance complexity.
• AGS, commonly applied in SBR systems, improves sludge settling characteristics and enhances nutrient removal. The granular structure allows simultaneous removal of organic matter, nitrogen, and phosphorus [7].

4. Sequencing Batch Reactor

The Sequencing Batch Reactor (SBR) is a fill-and-draw activated sludge where all treatment stages—equalization, aeration, and clarification—occur sequentially in one tank. This design offers flexibility and compactness, making SBRs suitable for municipal and industrial wastewater under variable flow conditions [5,8]. The SBR cycle typically involves five phases: fill, react, settle, draw, and idle. Each phase can be adjusted to achieve desired treatment goals, such as nutrient removal or improved sludge settling [5,9].

The operation of SBR consists of five steps:

• Fill: During the fill phase, incoming wastewater is introduced into the tank, mixing with the residual biomass from the previous cycle. This stage is typically occupying around 25% of the total cycle time [5].
• React: During the react stage, wastewater flow into the tank ceases, and biological degradation occurs under controlled aerobic, anoxic, or anaerobic conditions, generally taking 50% or more of the cycle time [8,10].
• Settle: Solids separation takes place under quiescent conditions (i.e., without inflow or outflow) in a tank, typically lasting 0.5 to 1.5 h [10].
• Draw (Decant): The draw phase uses a decanter to remove the treated effluent. The draw time typically ranges from 5% to more than 30% of the total cycle time [5,10].
• Idle: The period between draw and fill phases, often used for sludge wasting [9].

4.1. Advantages of SBR Systems

• Combines equalization, biological treatment, and clarification in one vessel, reducing equipment needs [8].
• Offers excellent operational flexibility and adaptability to changing inflow or load [5].
• Produces stable effluent quality with low suspended solids [11].
• Occupies a smaller footprint compared to conventional systems [8].

4.2. Applicability of SBR for Septage Treatment

A notable example of SBR application for septage is the Baliwag Water District’s septage treatment facility in Bulaca, Philippines [12]. Commissioned in 2013, this plant is designed to treat about 30 m³/day of septage, serving a population of roughly 28,000 households. The treatment includes pre-processing to handle the septage solids: bar screening and maceration, followed by primary screening with washing presses to remove solid particles, and a sludge thickening step with polymer to separate the solid fractions. After these steps, the liquid portion of the septage is held in an equalization tank to homogenize its quality before biological treatment. The core biological process is an SBR, which aerates the pretreated septage in batches to metabolize organics. The final effluent is stored and eventually used for irrigation or discharged safely, while the separated sludge is further dewatered and managed as a soil conditioner. This case demonstrates a successful integration of SBR in septage treatment.

5. Membrane Bioreactor

Membrane bioreactors (MBRs) integrate biological degradation and membrane filtration in a single process, producing high-quality effluent with minimal suspended solids and pathogens [6,13]. MBRs use microfiltration (MF) or ultrafiltration (UF) membranes to separate treated water from biomass. This complete retention of solids results in highly clarified effluent suitable for reuse in irrigation, industrial applications, or indirect potable supply [6].

Two main MBR configurations are widely implemented:

• Submerged MBRs, where membranes are immersed directly within the bioreactor. Suction pressure draws permeate through the membrane while aeration prevents fouling. This configuration is compact and energy-efficient, making it ideal for municipal wastewater [13].
• Side-stream MBRs, in which the membrane modules are located externally, and the mixed liquor is pumped through them at high velocity. Though more energy-intensive, this design allows easier maintenance and is preferred for industrial and high-strength wastewaters [6].

Operational parameters such as mixed liquor suspended solids (MLSS) concentrations (typically 8–12 g/L) and hydraulic retention time (HRT) (6–12 h) are optimized to balance treatment efficiency and membrane performance. Solids retention times (SRTs) are often extended (20–40 days), promoting complete nitrification and stable biological processes. Despite these benefits, MBRs face challenges such as membrane fouling, which occurs due to the accumulation of colloids, microbial products, or inorganic scaling. Fouling control involves maintaining adequate cross-flow velocities, regular aeration scouring, and periodic cleaning—either physical (air backwash and relaxation) or chemical (chlorine and citric acid) [14,15,16].

5.1. Advantages of MBRs

• Consistently produces reuse-quality effluent, free of bacteria and pathogens [17].
• Eliminates the need for secondary clarifiers and tertiary filtration [18].
• Smaller footprint and smaller reactor volume because of higher MLSS concentration and loading rate [13].
• Operates efficiently under longer solids retention times, enhancing the degradation of slowly biodegradable compounds [18].
• Allows complete biomass retention, reducing sludge washout [13].

5.2. Limitations

• High energy demand for aeration and recirculation.
• Membrane fouling increases operational and maintenance costs.
• Requires skill operation and regular monitoring to sustain performance.

5.3. Applicability of MBR for Septage Treatment

An example of MBR application is in the Seeley Country Water District [19], where they upgraded its lagoon-based facility in 2018 with a 0.95 MLD immersed hollow-fiber MBR to treat both the municipal sewage and hauled septage. Septage is first received at a dedicated station and buffed in the old lagoon before being gradually fed to the MBR. The system includes fine screening, grit removal, an aerobic bioreactor with submerged membranes, and UV disinfection. It consistently produces reuse-quality effluent. Key challenges—shock loads, high solids, and fouling—were managed by equalization, conservative membrane flux design, and robust air cleaning regimes. The upgrade brought the plant into compliance with stringent standards, and the effluent is earmarked for park irrigation, demonstrating that full-scale MBRs can effectively handle co-treatment of septage and sewage when supported by pretreatment and buffering.

6. Aerobic Granular Sludge

Aerobic Granular Sludge (AGS) technology represents one of the most advanced biological treatment processes developed for sustainable wastewater management. It uses compact, self-immobilized microbial aggregates—called granules—ranging from 0.5 to 3 mm in diameter to simultaneously remove organic matter, nitrogen, and phosphorus in a single reactor. These granules consist of mixed microbial communities embedded in a matrix of extracellular polymeric substances (EPS), which provide mechanical stability and resistance to toxic shocks [20,21].

AGS systems are typically operated in SBRs under alternating anaerobic, anoxic, and aerobic conditions. The short settling time selectively favors dense, well-settling granules over flocculent biomass. Within each granule, stratified redox zones develop—an aerobic outer layer and an anoxic or anaerobic core—enabling simultaneous biological oxidation, nitrification, denitrification, and phosphorus uptake. This internal structure allows the system to achieve high removal efficiencies even under varying load conditions [20,21].

Successful AGS formation requires strict control of settling time, feed strategy, and aeration intensity. Short settling time promotes granule selection, while feast–famine conditions encourage the growth of polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs), critical for phosphorus removal. Continuous monitoring of dissolved oxygen and intermittent aeration help maintain balanced redox conditions [20].

6.1. Advantages of AGS

• Reduces footprint by 50–75% compared to activated sludge [7].
• Achieved up to 50% energy savings through efficient aeration control [21].
• Allows simultaneous carbon, nitrogen, and phosphorus removal [20].
• Produces low excess sludge yields (20–30% reduction) [7].

6.2. Applicability of AGS for Septage Treatment

AGS technology, although relatively new, has shown excellent performance in treating high-strength wastewater, including septage from pit latrines and holding tanks. The case study of WWTP Búč (1600 PE) in Slovakia [22]—a plant that treats exclusively hauled septage—provides compelling evidence of AGS efficiency for decentralized septage management. This WWTP is achieving >94% removal of BOD, COD, and ammonia without chemical precipitation. Phosphorus removal efficiency reached 85% through enhanced biological uptake. The plant also demonstrated exceptional process stability under fluctuating influent loads, confirming the suitability of AGS for decentralized septage treatment.

7. Conclusions

Septage represents a highly variable and concentrated waste stream that poses significant challenges to treatment systems. The analyses of samples conducted in this study confirmed the large fluctuations in key parameters such as COD, BOD, nitrogen, and phosphorus, which highlight the need for adaptable treatment technologies.

Among the technologies reviewed, SBR offers operational flexibility and compactness, making it suitable for small-scale facilities dealing with intermittent septage inflows. MBR provides the highest effluent quality and pathogen removal, but their energy intensity and membrane fouling risks limit wider adoption, particularly in decentralized contexts. AGS technology demonstrates great promise, combining a compact footprint, high biomass retention, and simultaneous nutrient removal. The trial run carried out for AGS under septage conditions further demonstrated its resilience and applicability in real-world scenarios. To provide a concise overview of the discussed systems,

Table 3. Comparative summary of SBR, MBR, and AGS technologies.

Parameter	SBR	MBR	AGS
Process type	Fill-and-draw activated sludge	Biological + membrane filtration	Granular biomass in SBR mode
Main advantages	Flexible, compact, low cost	Excellent effluent quality, pathogen removal	Compact, energy-efficient, multi-nutrient removal
Limitations	Requires batch operation	Membrane fouling, high energy use	Startup control critical, limited full-scale data
Effluent quality	BOD5 < 20 mg/L; TSS < 30 mg/L	BOD5 < 5 mg/L; TSS < 1 mg/L	BOD5 < 10mg/L; COD < 30 mg/L
Energy demand	Moderate	High	Low-moderate
Suitability for septage	Proven in small systems	Effective with pretreatment	Highly promising (demonstrated)

presents a comparison of their main operational characteristics, advantages, and limitations.

Overall, the comparison shows that no single technology is universally superior; the choice depends on local conditions, regulatory requirements, and operational capacity. However, AGS emerges as a particularly promising option for decentralized septage management due to its robustness, lower energy requirements, and ability to achieve multi-pollutant removal in a single reactor.