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Practice Guide

Innovative Rural Wastewater Management

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4.2.1 Septic Tank

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PART I INTRODUCTION 
- 1 Introduction 
PART II INSTITUTIONAL AND REGULATORY FRAMEWORK 
- 2.INSTITUTIONAL, POLICY, REGULATORY FRAMEWORK FOR RURAL SANITATION AND WASTEWATER MANAGEMENT 
  - 2.1 Overview
  - 2.2.Institutional Arrangement 
  - 2.3.Policies and Regulations 
  - 2.4 Discharge Standards 
  - 2.5.Sources of funds 
  - 2.6.Typical provincial cases
  - 2.7.Conclusions and recommendations 
PART III TECHNICAL BASIS 
- 3 Overview of Rural Sanitation and Wastewater Management 
  - 3.1 Domestic Wastewater
  - 3.2 Rural Toilets in China – Source of Black Water 
  - 3.3 Decentralized vs. Centralized Rural Wastewater Management 
- 4 Rural Wastewater Treatment Technology 
  - 4.1 Preliminary Treatment 
    - 4.1.1 Bar Screens
    - 4.1.2 Grit Chambers
  - 4.2. Primary Treatment 
    - 4.2.1 Septic Tank
  - 4.3 Secondary Treatment 
    - 4.3.1 Attached Growth Process 
      - 4.3.1.1 Trickling Filter
      - 4.3.1.2 Rotating Biological Contactorr
    - 4.3.2 Suspended growth Process 
      - 4.3.2.1 Activated Sludge Process
      - 4.3.2.2 Sequencing Batch Reacto
    - 4.3.3 Waste Stabilization Pond
    - 4.3.4 Constructed Wetlands
    - 4.3.5 Subsurface Wastewater Infiltration Systems
- 5 Wastewater Treatment Process Design 
  - 5.1 General Design Consideration 
  - 5.2 Sewage Collection Alternatives 
  - 5.3 Wastewater Treatment Process Design 
  - 5.4 Water Reuse
  - 5.5 Sludge Management 
    - 5.5.1 On-site Sludge Management
    - 5.5.2 Off-site Sludge Management
PART IV PROJECT PLANNING AND DESIGN 
- 6 Project Planning and Design 
  - 6.1 Diagnosis for Project Villages – Initial Community Assessment
  - 6.2 Establishment of Stakeholder Group
  - 6.3 Assessment on Existing Conditions and Community’s Capacity 
    - 6.3.1 Physical Conditions Assessment
    - 6.3.2 Community’s Capacity Assessment 
      - 6.3.2.1 Project management capacity
      - 6.3.2.2 Financial capacity
  - 6.4 Baseline Engineering Survey and Assessment
  - 6.5 Project Feasibility Study and Environmental Impact Assessment
  - 6.6 Selection of Operation Model
  - 6.7 Project Cost Estimate
- 7 Community Participation 
  - 7.1 Why Need Community Participation?
  - 7.2 Principles of Community Participation
  - 7.3 Community Participation Activities 
    - 7.3.1 Community-based Organization Arrangement
    - 7.3.2 Public Health Promotion
    - 7.3.3 Project Planning and Design Phase 
    - 7.3.4 Project Implementation Phase
    - 7.3.5 O&M Phase
PART V PROJECT FINANCING 
- 8 Financing, Subsidies, and Cost Recovery 
  - 8.1 Programmatic Costs
  - 8.2 Project Implementation Costs 
  - 8.3 Project Financing 
    - 8.3.1 Alternative Sources of Funds
    - 8.3.2 Non-Monetary Costs
  - 8.4 Subsidies 
  - 8.5 Cost Recovery 
PART VI PROJECT IMPLEMENTATION AND MANAGEMENT 
- 9 Procurement and Implementation 
  - 9.1 Procurement Principles
  - 9.2 Procurement Alternatives 
  - 9.3 Procurement Planning 
- 10 System Adminstration, Operation, Maintenance and Monitoring 
  - 10.1 Introduction 
    - 10.1.1 Sustainability of A Rural Wastewater Management Project
  - 10.2 Management and Administration Arrangement
  - 10.3 Operation and Maintenance 
  - 10.4 Reporting and Monitoring 
  - 10.5 Operator Training and Support
Appendix: Case Studies – Rural Wastewater Management in Zhejiang, Shanxi, and Jiangsu Province 
- 1.Zhejiang Province 
- 2.Shanxi Province 
- 3.Jiangsu Province 
- 4.Summary 
  - 4.Summary
REFERENCES 
- 1.REFERENCES

4.2.1 Septic Tank

Categories: 4.2. Primary Treatment
Time of issue: 2022-04-28 18:23:16
Views: 0

Description:

Information

For households without connection to public sewer systems, a septic tank is usually used as the primary treatment unit in rural areas before subsurface infiltration systems or other secondary treatment processes. In China, a prefabricated concrete, fiberglass, PVC, or plastic septic tank is commonly applied.

Septic tank

It is commonly a covered and watertight rectangular or cylindrical tank with a top opening providing quiescent conditions to remove organic solids in wastewater through physical settlement and anaerobic digestion.

Figure 4.3 Schematic diagram of a two-chamber septic tank

(Source: istockphoto.com)

Treatment Process:

A septic tank involves processes of sedimentation, flotation, and anaerobic digestion. When raw wastewater enters the tank, the inlet tee slows down the flow rate and minimizes the turbulence in the tank. Wastewater is separated into a scum layer on the top, a liquid layer in the middle, and a sludge layer at the bottom in a septic tank.

Scum layer: low-density materials (such as oil and grease) rise to the surface of wastewater in tank and form a scum layer. It helps create an anaerobic environment in the tank.
Liquid layer: clarified liquid in the middle layer is discharged to leach field or other secondary treatment units before disposal.
Sludge layer: suspended particles, including bacteria and parasite ova, are flocculated into larger aggregates and settle at the bottom of the tank as sludge. Anaerobic digestion occurs in this layer to breakdown organic matters into methane (biogas) and carbon dioxide, etc., which is released to the atmosphere through a ventilation pipe.

Treatment Efficiency:

Pollutant/Parameter	Removal efficiency
BOD	30%-50% (UNEP, 2004) (Note: The treatment efficiency is summarized from literature review. Based on the practical experience of the authors, the septic tank is expected to remove 10%-15% of BOD in raw domestic wastewater at most if operating in good condition.)
Suspended solid	40%-60%
Note: The treatment efficiency varies significantly depending on the influent concentrations, climatic conditions, and design and maintenance of the septic tank.

Source: UNEP 2004

Design Criteria:

When selecting the location for the septic tank, the following considerations should be taken into:

the site should be an open area to avoid extra vertical load (e.g., vehicular traffic) to the buried tank and has a flat and stable base to prevent subsidence.
the site should be accessible for routine septage removal.
when slope exists, a stormwater diversion drain and/or a surface drainage berm built with mounded soil should be installed to prevent pooling around the septic tank. Septic tanks should not be installed in flood plains, drainage ways or depressions unless flood protection is provided.
the location should be at least 5 m away from the house.
the location should be at least 30 m away from drinking water wells to minimize the risks of water source contamination.
the location should be at least 3 m away from large trees.

The tank shall be set on undisturbed soil that allows a 2.5% slope in the building sewer. If the natural soil is soft, 30 cm of soil should be replaced with crushed rock to bed the tank on firm material.
The tank shall meet appropriate strength requirements to withstand the exterior soil pressures and interior liquid pressures.
If local groundwater level is high, ground anchors may be applied to fix and secure the tank in the ground to prevent flotation when the tanks are emptied. Concrete tanks are recommended for areas with shallow groundwater.
The area around the tanks should be suitably vegetated with non-invasive and local plants to minimize soil erosion from the excavated area.
The minimum depth of backfill on the tank is 300 mm. Backfill should be free of large stone, rubbish, saturated soils, or other unsuitable materials.
The USEPA recommends the minimum septic tank capacity of 1000 gallon, (equals to 3,785 L, or 3.78 m3) for individual dwelling. In China, the effective depth, width, and length of a septic tank shall be no less than 1.3 m, 0.75 m, and 1 m, respectively, equals to a minimum volume capacity of approximately 1 m3 as specified in the CJJ124-2008. For a circular septic tank, the diameter shall be no less than 1 m.
Larger tanks have the capacity to retain more wastewater, sludge, and scum, which results in longer HRT, improved sedimentation and flotation performance, more efficient anaerobic digestion, and less frequent pumping requirements. Although larger tanks have higher capital costs, the operation and maintenance costs are lower due to less frequent pumping.
The designed HRT in a septic tank shall be at least 45 hours.
The outlet invert of the tank should be at least 50 mm below the inlet invert. A drop of 75 mm is recommended.
The inlet to the septic tank should have a flow diversion (a baffle or tee) device extending not less than 75 mm into the sewage. The rising inlet of the tee shall extend at least 150 mm above the liquid level to prevent the scum layer from plugging the inlet.
The outlet of the septic tank should have a baffle or tee fitting extending 450-650 mm into the sewage. The top of the fitting or baffle should not be more than 50 mm below the ceiling of the tank.
A minimum space above the liquid level of 300 mm should be provided for scum storage and ventilation.
Manways shall be provided over each chamber of the tank to allow for removal of accumulated sludge and scum. An inspection port should be installed above the inlet and outlet to allow the sludge and scum levels to be routinely checked to determine when solids removal is necessary.
Septic tank performance is significantly improved by including an effluent outlet filter on the septic tank outlet. These filters minimize the solids carry-over to the land application system.

For a three-chamber septic tank, as required in GBT 38836-2020:

The total effective volume shall be no less than 1.5 m3. The volume capacity ratio among three chambers shall be 2:1:3.
The solid retention time (SRT) shall be no less than 20 days and 10 days in the first and second chambers, respectively, while the SRT in the third chamber shall be no less than the total SRT in the first two chambers.
In cold areas, the buried depth of the septic tank shall depend on the depth of frozen soil layer.
The inner diameter of inlet pipe shall be no less than 100 mm with a slope no less than 20% and a horizontal distance less than 3 m. If the horizontal distance exceeds 3 m, the slope of pipe shall be increased accordingly.
The transfer pipes between the chambers shall be designed as reverse L-shape or I-shape with an inner diameter of at least 100 mm. The distance between the transfer pipes and the top of the tank shall be no less than 100 mm. The inlet of transfer pipes connecting 1st-2nd chambers and 2nd-3rd chambers shall be designed to have a minimum distance of 1/3 and 1/2 of the tank’s effective depth to the bottom of the tank, respectively.
The ventilation pipe shall be installed in the first chamber with a minimum inner diameter of 100 mm. The top of ventilation pipe shall be designed as tee-shape to protect pipe from rain.
An access opening with a minimum diameter of 200 mm shall be designed on each chamber for routine inspection and sludge cleaning. When the diameter is larger than 250 mm, lids shall be secured to prevent accidental entry and falling. The lids shall be at least 100 mm above final ground level to prevent stormwater from entering the tank.

Key Design Parameter:

The sizing of septic tank influences the treatment performance. Larger septic tanks provide longer hydraulic residence time but require higher capital cost. The size of a septic tank will be determined based on serving population, wastewater flow, wastewater composition, hydraulic residence time, storage volumes for sludge and scum, and solid cleaning frequency. Table 4.1 provides recommended volume of a septic tank in different types of villages. The estimation is based on the following assumptions:

wastewater discharge coefficient is 80%;
5 people in each household;
the hydraulic residence time is 45 hours;
0.8 L of sludge generated per capita per day;
sludge cleaning period is once per year;
the moisture content of fresh sludge is 95%;
the reduction of sludge volume after anaerobic digestion is 20%;
the moisture content of digested sludge is 90%.

Table 4.1 Recommended volume of a septic tank in different types of villages

Type of Villages	With flush toilet, and With shower facilities	With flush toilet, and Without shower facilities	Without flush toilet, and Without shower facilities
Estimated Maximum WW Generated (L/capita/day)	200	130	50
Estimated volume of WW per HH per day (m3)	1.88	1.22	0.47
Estimated volume of sludge per HH per day (m3)	0.58	0.58	0.58
Recommended volume of a septic tank for each HH (m3)	2.46	1.8	1.5

Installation Consideration:

Proper pipe and tank bedding is important to ensure that shearing of the pipe at the tank inlet, outlet or at the foundation does not occur. Figure 4.4 shows different tank installation and excavation scenarios. An improperly supported septic tank will settle and may cause breakage or disconnection of the inlet and outlet piping from the tank. The long sloping ends of an excessive excavation cannot provide support for the inlet and outlet piping connected to the septic tank. The weight of the soil above the unsupported piping may cause the breakage of the piping.

Figure 4.4 Excavation and installation of septic tanks

(Source: Government of Saskatchewan, 2018)

The septic tanks need to be installed as per the manufacturer’s specifications. Bedding material in the excavation will need to be compacted and leveled. The installer should take care that the tank remains undamaged and that the manufacturer’s instructions for back filling are followed.
Septic tank should not be buried deeper than the depth recommended by the manufacturer. The excavation necessary for a septic tank may be reduced if advantage is taken of natural ground surface grade.
A septic tank shall have adequate earth cover or other means to protect it from freezing during operation in cold winters.
Where a septic tank is installed in areas of high groundwater conditions, the system shall: a) ensure that the water table does not arise above any openings to the tank; b) anchor the tank per manufacturer’s instructions to prevent floatation and/or shifting; and c) maintain the elevation of the piping connections above the projected water table or include other specific additional measures to ensure that infiltration does not occur through piping connects or manhole risers.

Operation and Maintenance:

The septic tank is a passive treatment unit and requires little intervention during operation. During operation, operators shall pay attention to the following issues:

Conducting routine inspection on the septic tank to check a) if the tank is watertight and structurally sound with no corrosion or cracking; b) whether the inlet and outlet tees and baffles are in good condition with no blockages; c) whether the thicknesses of sludge and scum are in a reasonable range.
The sludge and scum shall be regularly removed (normally every 12 to 36 months) using a septage pumping system. Generally, sludge cleaning is needed when the total depth of sludge layer and scum layer reaches 50% of the tank depth.
Effluent outlet filters require regular washing to remove solids and algae build up and should be inspected at least six monthly and cleaned as required.
Before sludge cleaning, a stirrer should be used to completely mix the sludge, scum, and liquid in the tank.

Capital Cost and O&M Cost:

Capital cost. The capital cost for septic tanks is low. Key factors influencing the capital cost include subsurface site conditions, accessibility to the site, material of the tank, size of the tank, and the depth of excavation. Where vehicular traffic is expected, deeper installation will be required, which is more expensive. Table 4.2 provides indicative expense for a single-chamber septic tank construction. The actual costs will vary significantly depending on site conditions and local economics.

Table 4.2 Indicative expense for brick and prefabricated reinforced concrete septic tank

Tank volume (m3)

1.8

2.5

100

Brick septic tank

(thousand CNY)

1.7

2.1

13.7

14.2

19.8

25.1

30.8

62.7

Prefabricated reinforced concrete septic tank

(thousand CNY)

8.2

12.3

16.4

21.3

25.0

49.3

Source: Regional Rural Domestic Wastewater Treatment Technical Guide (2010, on trial)

O&M cost. The operation cost of septic tanks is low. Neither electrical energy nor professional technician is required. The maintenance cost depends on the frequency of septage removal and the charge of service provider.