Lecture 6bSewage Treatment & Constructed

Wetlands

Using Wetlands for sewage treatment. By Jennie Swenson & Terry Cooper

http://agen521.www.ecn.purdue.edu/AGEN521/epadir/wetlands/graphics.html

On Site Sewage Treatment Systems – Septic Tank Systems

1855-First U.S. sewage treatment system

• The wastes generated by some 60% of the U.S. population are collected in sewer systems and carried along by some 14 billion gallons of water a day.

• Some 10% is allowed to pass untreated into rivers, streams, and the ocean.

• The rest receives some form of treatment to improve the quality of the water (which makes up 99.9% of sewage) before it is released for reuse.

Untreated sewage discharge is a persistent problem that seems to be getting worse in an era of regulatory neglect. http://enr.construction.com/features/_gallery/050817/050817-5.asp

Sewage Treatment Technology• Saved more lives than

any other technological development

• A sewage treatment plant is nothing more than a LARGE MICROBIAL CULTURE FLASK

• The result of this process converts most of the nutrients to chemicals like carbon dioxide, nitrate, sulfate, phosphate; i.e., minerals

• Raw sewage is rich in organic nutrients such as human excrement, and food and industrial wastes.

• Since microbes grow and utilize nutrients most efficiently under AEROBIC CONDITIONS, sewage treatment plants are designed to provide excess OXYGEN for the microbes.

Hong Kong Sewage Treatment Plant

• Finally, there is always some material that can not be easily degraded by microbes which SETTLES OUT at various stages in the treatment process.

• This material is called SLUDGE and it must also be disposed of as part of the sewage treatment process.

Land Spreading Ocean Dumping

Mirfield Sewage Sludge Incinerator -UK

Fertilizer

Alternative Sewage Technology

•Constructed Wetlands

– Engineered system

– Utilize natural processes

– Treat wastewater

Constructed wetlands are small artificial wastewater treatment systems consisting of one or more shallow treatment cells, with herbaceous vegetation that flourish in saturated or flooded cells. They are usually more suitable to warmer climates. In these systems wastewater is treated by the processes of sedimentation, filtration, digestion, oxidation, reduction, adsorption and precipitation.

3-System Designs 1)Subsurface

Flow System

2)Free Water Surface

3)Aquatic Plant SystemThe Water holding

structure is constructed in basin

or channel. Some form of subsurface

barrier limits seepage in first

basin- even a wet soil can be used.

Subsurface Flow System (SFS)

•Water flows below media-

No water on soil surface but subsoil is saturated•Sand, gravel, rock

•Grasses, trees

•Minimal land

Subsurface Flow System

Free Water Surface (FWS)

•Water flows over soil media

•Water <18”•Sedges, reeds, rushes•Land intensive

Free Water Surface

Aquatic Plant System (APS)

•Similar to FWS•Water >18” •Water hyacinth,

duckweed, pennywort

•Fish

Aquatic Plant System

Constructed Wetland Scales

Subsurface Flow Free Water Surface

Major Mechanisms of Pathogen Removal

• Sedimentation• Predation

• Adsorption • Inactivation

Bacteria ->

Viruses ->

Percent RemovalFecal Coliform

# Type Range Avg

29 Subsurface 99.9 - 78.2 97.6

8 Free Water 99.9 - 81.6 91.8

4 Aquatic Plant 98.5 - 43.2 79.5

Reasons Cited for High Removal Rates

•Long retention time •Low effluent loading rate•Vegetation

– Increase microbial population – Root excretions– Aeration of media

Reason Cited for Low Removal Rates

•Insufficient sunlight•Lack of maturity•Excessive wildlife•High turbidity

–Resuspension of solids–Water soluble humic substances

Spring Hill’s Wastewater System – Innovative Technology

• Description: The City of Spring Hill, population 77, had nonconforming septic tanks connected by a community sewer that ultimately discharged to the Sauk River without further treatment.

• The unauthorized discharge needed to be corrected, but the cost of compliance was of great concern.

• Solution: Spring Hill’s new wastewater treatment system consists of a subsurface flow constructed wetland followed by disposal by drip irrigation. The treatment system is capable of treating 9,200 gallons per day of domestic wastewater.

• The construction cost of the treatment and disposal system was approximately $285,000. The sewage collection system, designed by the city engineer, added another $310,000 to the total capital cost of the system.

• The original Preliminary Engineering Report recommended regionalization at a capital cost of $805,000 plus approximately $200,000 of improvements at the regional pond system. The cost of the original plan, at over $25,000 per connection, was beyond the City’s financial capability.

• With the application of constructed wetland technology, the costs became affordable.

Cross section of Spring Hill wetland treatment cell plan

gravel

mulch

List of plants include:broadleaf cattail (Typha latifolia), hardstem bulrush (Scirpus acutus), river bulrush (Scirpus fluviatilis), duck potato (Sagittaria latifolia), wild iris (Iris versicolor), big bluestem (Andropogan gerardi), switchgrass(Panicum virgatum),

adaped from Widseth Smith Norlting and Associates report dated 11/98

rock

Wetland Cell Typical Cross Section

Inflow fromseptic tanks Outflow to

lift stationand drip irrigation

Dyad Problem:

• Calculate the amount of soil erosion from this 3 acre field in tons per acre.

• BD = Mass/Vol• BD soil = 1.33 g/cc• Area of soil measured = 30 ft x 40 feet. The soil was 8 inches

thick over this area.

• Solution • 1.33 x 62.4lbs/ft3 = 83lbs/ft3 & 30x40x8/12 = 800ft3

• 83s/ft3 = wt/800ft3 = wt = 83x800 = 66400 lbs/3acres• = 22133 lbs./acre or ÷2000lbs/ton = 11 tons/acre or 2 x sustainable rate- of 5 tons / acre

note: some soil did leave the field and was not in our calculation, I wonder how much???