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Practice Guide

Innovative Rural Wastewater Management

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4.3.4 Constructed Wetlands

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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

4.3.4 Constructed Wetlands

Categories: 4.3 Secondary Treatment
Time of issue: 2022-04-28 18:31:34
Views: 0

Description:

Information

Constructed Wetlands (CW)

Engineered of consdtructed wetlands that utilize natural processes involving wetland vegetation, soils, and their associated microbial assemblages to assist, at least partially, in treating an effluent or other water source.

Treatment Process:

In a constructed wetland, water slowly flows from the inlet to outlet, and the degradation of organic matters and nutrients in water occurs during its contact with aquatic vegetation and microorganisms. The bottom of constructed wetland is lined to prevent leaching of wastewater into surrounding soil and/or groundwater. There are typically two types of constructed wetland: the free water surface constructed wetland and the subsurface flow constructed wetland.

Free water surface constructed wetland (FWS-CW): water flows horizontally from inlet to outlet with water surface exposed to the atmosphere and sunlight. A soil layer is at the bottom of the wetland as rooting media for aquatic plants. Microorganisms grow attached to the roots or stems of aquatic plants to adsorb nutrients and degrade organic matters in the water. FWS are commonly used to achieve high removals of organic matter, suspended solids, and nutrients, most notably nitrogen.

Figure 4.11 Schematic diagram of a free water surface constructed wetland

(Source: Tilley et al, 2014)

Subsurface flow constructed wetland (SF-CW): a layer of porous media, usually consists of rocks or gravels or sands, is filled at the bottom of the wetland. It not only provides support to the root structure of emergent vegetation, but also acts as a filter with great surface areas for attached microorganisms to remove pollutants. The water level remains below the surface of rocks or gravels media in the bed.

According to the direction of water flow path, SF-CW can be further divided into horizontal flow and vertical flow. The vertical flow creates aerobic conditions for the biological degradation processes, while anaerobic conditions prevail in horizontal flow CWs.

Figure 4.12 Schematic diagram of a horizontal subsurface flow constructed wetland

(Source: Tilley et al, 2014)

Water Quality of Influent:

Type of CW

BOD₅

(mg/L)

COD_cr

(mg/L)

NH₃-N

(mg/L)

FWS-CW

≤50

≤120

≤100

≤15

≤3.5

SF-CW

≤80

≤200

≤70

≤25

≤5.0

Source: Technical Specification for Natural Treatment of Wastewater Engineering (CJJT 54-2017).

Treatment Efficiency:

Type of CW	BOD₅	COD_cr	SS	NH₃-N	TP
FWS-CW	40-70%	50-60%	50-60%	20-50%	35-70%
Horizontal SF-CW	45-85%	55-75%	50-80%	40-70%	70-80%
Vertical SF-CW	50-90%	60-80%	50-80%	50-75%	60-80%

Source: Technical Specification of Constructed Wetlands for Wastewater Treatment Engineering (HJ 2005-2010).

Design Criteria

Criteria	FWS-CW	SF-CW
Length to width aspect ratio	3:1 to 5:1	1:1 to 5:1
Bottom slope	0.4 to 0.5% The top slope should be nearly level	0.5 to 1.0% The top slope should be nearly level
Pond configuration	at least two ponds in series	at least two ponds in parallel
Vegetation	Emergent plant and submergent plant	Emergent plant

For free water surface constructed wetland:

The recommended minimum hydraulic residence time in the wetland for reduction of organic matter and nitrogen is 7 days.
The bottom of the ponds should be lined with a clay or synthetic liner to prevent leakage. At least a 30 mm liner should be used.
The pond embankment slopes should be not steeper than 2 horizontal to 1 vertical.
At least 0.6 m of freeboard should be maintained when the pond water level is full.
At least 0.5 m of soil should be replaced on over the liner to support the roots of the wetland plants.
The wetland should be configured with both shallow (0.5 to 0.9 m) and deep zones (1.2 to 1.5 m). Emergent vegetation should be planted in the shallow zones and submergent plans should be planted in the deep zones of the ponds. The ratio of shallow to deep zones should be 1:1.
The first shallow water zone should be sized to provide a HRT of 2 days to improve liquid-solid separation. To help reduce excessive growth of algae in the open water zones, the HRT in the deep-water zones should be maintained between 2 to 3 days. The remaining HRT in the shallow zones should be 2 days.
The inlet and outlet pipes of the pond should be connected to a perforated pipe manifold installed on the bottom of the pond. The pipe manifold is made of a perforated PVC pipe with a diameter of 100 mm to 150 mm. The width of the pipe manifold should be equal to the width of the pond on each end.
The outlet pipe of the pond should be connected to a water level control structure, such as small weir box. The weir box is used to adjust the water level in the wetland. The outlet weir should be sized to maintain the weir overflow rate below 200 m³/m•d.

Local aquatic plants with wide and deep roots should be selected. Common emergent vegetation is Typha or Scirpus. Common submergent plants include Potamogeton and Elodea.
No trees around the selected site as the tree roots may harm the liner.

If required, plants can be harvested every 3 to 5 years.

For subsurface flow constructed wetland:

Even cross-sectional flow throughout the wetland is important to ensure the treatment efficiency.
Small, round, evenly sized gravel is most used to fill the bed to a depth of 0.5 to 0.6 m. Media should be resistant to crushing or breakage.

Zones	Diameter of media
Inlet Zone (first 2 meters)	40 – 80 mm
Treatment Zone	20 – 30 mm
Outlet Zone (last 1 meter)	40 – 80 mm
Planting media (top 10 cm)	5 – 20 mm

The recommended depth of the water is 0.6 to 1.6 meters for horizontal SF-CW, and 0.8-2.0 meters for vertical SF-CW. The water level should be maintained at 5 to 15 cm below the surface to ensure subsurface flow.
At least 0.3 m of freeboard should be maintained.
The minimum length of the wetland should be 15 meters.
Perforated pipes shall be evenly distributed at the surface or the bottom of the media layer. The flow rate in the perforated pipes shall be 1.5 m/s to 2.0 m/s. The pore size shall be 5 mm to 10 mm with flow rate no less than 1m/s.
Use adjustable inlet and outlet device with capability to balance flows and control water level in the wetland.

Key Design Parameter:

The design parameters shall be determined on the basis of local climate conditions, experimental data analysis, or operation experiences from other similar constructed wetlands. The following five tables listed reference data for key parameters for constructed wetlands at different temperature.

Table 4.5 Key design parameters for constructed wetland in severe cold area

(average temperature≤-10℃ in January and ≤25℃ in July

No. of days with daily average temperature≤5℃ ≥145 days)

Design parameter	FWS-CW	Horizontal SF-CW	Vertical SF-CW
Hydraulic retention time (d)	3.0-20.0	2.0-5.0	1.5-4.0
Surface hydraulic load (m³/m²•d)	0.01-0.1	0.2-0.5	0.3-0.8
COD area loading rate (g/(m²•d)	0.1-5.0	1.0-10.0	1.5-12.0
NH₄-N area loading rate (g/(m²•d)	0.01-0.20	0.5-2.0	0.8-3.0
TN area loading rate (g/(m²•d)	0.02-2.0	0.4-5.0	0.6-6.0
TP area loading rate (g/(m²•d)	0.005-0.05	0.02-0.2	0.03-0.2