Low Cost WW Treatment

EFFECTIVE WASTEWATER TREATMENT AT LOW CAPITAL AND OPERATING COSTS

W. K. (Tim) Journey, Senior Associate, EarthCAD, Inc.

Most municipal governments aspire to high sanitary service levels, but the high cost of building and operating conventional mechanical wastewater treatment plants is frequently cited as a reason for not treating wastewater before its disposal. This discussion describes wastewater treatment technologies whose capital and operating costs are lower by an order of magnitude than those of “conventional” mechanical treatment.

I. High rate, high power consumption process: mechanical aerobic wastewater treatment

Mechanical treatment plants are high rate biological reactors capable of achieving a secondary quality final effluent by using various types of mechanical equipment to mix the substrate with the bulk liquid as the means to supply oxygen to the aerobic bacteria that biodegrade organic wastes. Aerobic treatment may also be designed to support the biological processes of nitrification and denitrification that remove nitrogen and phosphorus. Mechanical aeration distributes oxygen more efficiently than naturally aerated systems, such as lagoon systems. Efficient aeration reduces the volume of the treatment plant, and the final effluent is low in BOD5 (<10 mg/l). For over 60 years, mechanical aerobic treatment technologies have been used to treat municipal effluents and dilute industrial wastewaters. One of the most popular aerobic processes is the activated sludge process and its recent variants, including sequencing batch reactors, extended aeration and the oxidation ditch. In many developing countries, mechanical treatment technologies have important disadvantages besides high investment cost: high power consumption, vulnerability to power outages, high maintenance requirements and the need for close supervision by skilled operators to avoid process upsets.

II. High rate, low power consumption wastewater treatment process: the UASB anaerobic reactor

The upflow anaerobic sludge blanket (UASB) reactor is also a high rate wastewater treatment process that relies on bacteria that break down organic material in the absence of oxygen. The UASB reactor was pioneered in India and Colombia in the 1980s and is now popular in regions with warm climates. A normally functioning UASB reactor removes an average of 65 percent of chemical oxygen demand (COD), 80 percent of biological oxygen demand (BOD5) and 75 percent of total suspended solids (TSS) from a typical raw municipal wastewater. The resulting effluent quality will be intermediate between primary and secondary (30-70 mg/l for BOD5 and 40-50 mg/l for suspended solids) and generally needs further treatment (polishing) before final disposal. Initial anaerobic treatment of wastewater in a UASB reactor has important advantages over mechanical aerobic processes:1

o Low investment cost (from 1/5 to 1/10 of the cost of mechanical aerobic treatment);

o Efficient treatment of high-strength waste streams without externally supplied power;

o Construction primarily from materials used to build housing, such as concrete and masonry;

o Modular design, infinite scalability and linear cost increases as the size of the plant increases;

o Few moving parts, minimal equipment;

o Plant operators do not need high-level academic qualifications;

o Surges in organic loading (shock loads) handled better than aerobic systems;

o Large flow variations and prolonged shutdown do not cause process upsets;

o Organic nutrients are converted into mineralized forms ideal for plant uptake;

o Low amount of residual sludge byproduct that has good settling properties, is easily dewatered and needs no additional treatment;

o Biogas generated by fermentation may be an economical fuel in large scale treatment plants;

o Refractory organic compounds are attenuated or degraded and heavy metals are removed;

o Organic material in wastewater is completely degraded to humus, CO2 and methane.

The polishing process for an anaerobic effluent should be designed to improve the effluent quality in the following parameters: pathogen contamination, residual organic material (COD/BOD5), oxygen demand from the reduced forms of N and S, residual suspended solids and the inorganic nutrients (N and P). Ideally, the polishing process for an anaerobically treated effluent should be aerobic because oxidative processes complement the reductive anaerobic processes. Linking the two types of processes in that order in a “treatment train” is the most efficient way to achieve complete biodegradation of organic material. The means of aeration may be either mechanical, as in the activated sludge, oxidation ditch or sequencing batch reactors processes, or passive, as in constructed wetlands or a series of facultative and maturation lagoons.

III. High rate, moderate power consumption treatment train: anaerobic initial treatment in series with mechanical aeration for final effluent polishing

Where skilled manpower and a reliable electric power supply are available, but not enough affordable land for waste stabilization ponds, there is an alternative that consumes less power than a “stand-alone” mechanical aerobic treatment plant. A UASB anaerobic reactor is used for initial treatment and mechanical aeration is used to polish the anaerobic effluent to an advanced secondary quality final effluent. Figure 2 compares the characteristics of the two options.

Research by Dr. Adrianus van Haandel in Brazil confirms that placing an anaerobic reactor upstream of a mechanically aerated treatment process has several important advantages over stand-alone aerobic treatment:2

o The volume of the anaerobic/aerobic treatment train will be about half that of a conventional activated sludge plant, reducing the capital cost of the treatment plant correspondingly.

o The demand for electric power for mechanical aeration to polish the effluent is reduced by more than half, reducing operating costs correspondingly and reducing by half the cost of mechanical and electrical components that would be required for a stand-alone mechanical treatment plant of the same capacity;

o The UASB reactor substitutes for primary treatment, a standard initial treatment process used in a mechanical treatment plant, eliminating that process and associated infrastructure from the treatment train of the hybrid plant;

o Sludge handling accounts for half to one third of the operating costs of an activated sludge treatment plant. The hybrid plant reduces sludge handling costs to insignificance by sending excess aerobic sludge from the polishing unit to the UASB reactor, where it is stabilized and densified.

o The cost of expanding the capacity of an overloaded mechanical treatment plant can be lowered significantly and the final effluent quality improved by installing UASB reactors upstream of the existing treatment plant instead of adding to the aerobic mechanical treatment capacity.

150 l/capita

19 l/capita settlerUASB

reactor25 l/capita reactor

14.5 l/capita

activated

influent

stabilized sludge0.25 l/capita

generatorbiogas

electricpower

sludge

excess sludge

Anaerobic/aerobic treatment plant

Figure 1. Comparison of volume per person served of wastewater treatment plant options: “stand-alone” aerobic versus anaerobic/aerobic

activated sludgereactor

43 l/capita

thickenerbiogas

19 l/capita

return sludge

influent

150 l/capita

8.6 l/capita stabilized

1.1 l/capita

digester32

l/capita

settler

sludge

secondary quality effluent

Stand-alone aerobic treatment plant

sludge

sludge advanced secondary

quality effluent return sludge

Figure 2 compares the fate of organic matter in a wastewater effluent treated in a “stand-alone” aerobic treatment process, or a treatment train in which the anaerobic process is in the first position. Note that the stand-alone aerobic treatment plant stabilizes half the organic matter by oxidation, while the anaerobic/aerobic treatment train oxidizes less than one fourth. In contrast, initial anaerobic treatment passively transforms half the organic matter into methane, reducing the amount of unstabilized organic matter that the aerobic process is called upon subsequently to oxidize. Also, the residual organic material in the effluent from the anaerobic/aerobic treatment train is half that of the stand-alone aerobic option, and a superior effluent quality is achieved at less than half the energy cost. Anaerobic/aerobic treatment uses both energy and investment capital more efficiently than a mechanical treatment plant in isolation.

IV. Completely passive wastewater treatment train: UASB initial treatment and final effluent polishing in a “constructed wetland”

Where land is available at an affordable price and/or an advanced quality (tertiary) final effluent is desired because of the sensitivity of the receiving waters, anaerobic initial treatment may be sequenced with a constructed wetland to polish the effluent. A constructed wetland mimics a natural marsh, which is characterized by grasses, other emergent macrophytes and/or floating aquatic plants that together function as a “biological filter”. Quiescent hydraulic conditions foster sedimentation of residual suspended solids, bacterial action degrades oxygen consuming substances, while plant growth assimilates the mineralized nutrients ammonium (NH4

2+) and orthophosphate (PO54-) that are of concern in wastewater treatment.

Polishing of anaerobic effluents in wetlands takes advantage of treatment processes that are entirely passive, and only simple farm implements are needed to harvest the plant biomass that constitutes the sink. Constructed wetlands are simple to build, and may be operated by personnel with farming skills. 1

Like anaerobic reactors, the budget for operating constructed wetlands is consists primarily of labor costs.

Vetiver grass (Vetiveria zizaniodes) is a fast-growing, natural hydrophyte that is at home in a marsh environment. Vetiver grass was taken from its home in India to many tropical countries by colonial agriculturalists and immigrants and is today a pan-tropical plant. However, it does not produce viable seed, is not invasive and is incapable of becoming a weed. Vetiver grass grows rapidly in constructed wetlands and is an effective biological filter to polish pre-treated municipal wastewater effluents. The resulting high quality final effluent may be reused when less than potable quality water is acceptable, for example, for many industrial applications, flushing of toilets and horticulture irrigation. Reuse of treated effluent effectively prevents residual pollutants from reaching receiving water bodies, while conserving a significant fraction of the primary water resource for higher value uses. Figure 3 illustrates the input-output relationships of a constructed wetland based on vetiver production. For purposes of illustration the example assumes initial concentrations of N and P in the effluent are 45 and 4 mg/l respectively.

in effluent10%

Methane gas20%

sludge

oxidized

50%

20%

50%

methane gas

5%

23%

sludge

oxidized

22%

in effluent

Stand-alone aerobic treatment Anaerobic/aerobic treatment

Figure 2. Comparison of the fate of organic matter in a“stand-alone” aerobic versus anaerobic/aerobic wastewater treatment

Figure 4 shows vetiver grass that was grown hydroponically on a wastewater effluent in tropical Australia in an experiment to assess nutrient removal ability. The most common method of treating industrial wastewater in Queensland is by land irrigation of tropical and subtropical pasture plants. A series of research projects conducted at a gelatin factory and at an abattoir determined that vetiver grass was superior to pasture grasses used for wastewater disposal.3 Vetiver grass has the potential to produce up to 132 tonnes per hectare per year (t/ha/year) of dry matter compared to 23 and 20 t/ha/year for Kikuyu and Rhodes grass respectively. This production level of vetiver can export up to 1,920 kg/ha/year of N (nitrogen) and 198 kg/ha/year of P (phosphorus), compared to 687 of N and 77 kg/ha/year of P for Kikuyu and 399 of N and 26 of P for Rhodes grass respectively. Vetiver growth can respond positively to a supply of N up to 6,000 kg/ha/year, and, to ensure this extraordinary growth and uptake of N, the P supply level should be at 250 kg/ha/year.3

Vetiver grass grown on wastewater effluent is a soft and palatable forage for animals with high nutritional value. Such fodder has up to two percent N, which is equivalent to 12.5 percent crude protein, more than some tropical legumes.4

Endnotes1 Journey, W. K. and S. McNiven. 1996. “Anaerobic Enhanced Primary Treatment of Wastewater and Options for Post-Treatment”. USAID Global Environment Bureau. Washington, DC.2 Van Haandel, A. and G. Lettinga. 1995. Anaerobic Sewage Treatment: A Practical Guide for Regions with a Hot Climate. John Wiley & Sons. New York.3 Truong, P., and R. Ash. (in draft). “The Use of Vetiver Grass Wetland for Sewerage Treatment in Australia”. To be presented in Oct. 2003 at the Third International Conference on Vetiver in China.4 Dr. Paul Truong. 2002. Personal communication.

150 l/capita

UASB

reactor

25 l/capita

influent

stabilized sludge0.25 l/capita

generator

biogas

electricpower

Figure 3. Anaerobic/aerobic treatment plant and final effluent polishing in a constructed wetland

Constructed wetland for final

Volume – 2 m3 per capita

effluent polishing (nutrient removal)

13 days hydraulic retention time

tertiary quality treated effluent:

Surface area –2 m2 per capita

complete removal of N & P

removal of pathogens

enhanced primary effluent

26 kg (dry weight)of vetiver grass per

capita per year

Figure 4.Root length of 780 mm after

three months growth on effluent. (Dr. Paul Truong)

Annex 1: Abstracts of Selected Vetiver Research Papers

The Use of Vetiver Grass Wetland for Sewerage Treatment in Australia

Ralph Ash* and Paul Truong**

* Utilities Engineer, Esk Shire Council, Esk, Queensland, Australia** Veticon Consulting, Brisbane, Queensland, Australia

Abstract: The Esk Shire Council has recently installed a Vetiver Grass System wetland to treat sewerage effluent at Toogoolawah in South East Queensland. The sewerage treatment plant is situated on a 22-hectare site on the northern edge of town.

The aim of this scheme was to improve water quality before the effluent discharges to the natural wetlands. The biggest problem with the quality of the effluent is its high nutrient loading. With the recent changes to license conditions imposed by the Environmental Protection Agency, the existing treatment plant no longer complies with the license and an upgrade of the plant was required.

Instead of the above set up, a new and innovative phyto-remedial technology recently developed in Queensland by the Department of Natural Resources and Mines, is being implemented at Toogoolawah. Under the Vetiver Wetland System, the effluent is being treated in two stages:

Preliminary treatment of the pond effluent in situ by floating pontoons placed in the ponds, and by vetiver planting around the edges of the three sewerage ponds.

Main treatment by vetiver wetland, once the effluent exits the sewerage ponds it passes through a Vetiver Grass contoured wetlands constructed over 3 hectares of the land. The Vetiver Grass wetlands have been constructed in rows following the contours to allow good contact between the grass and the effluent. The Vetiver Grass takes up the water and in particular, the grass will remove the nutrients from the water that passes through it.

As Vetiver Grass system is very effective in removing nutrient loads, it is expected that once the wetland is properly established there should be no release of sewerage effluent from the treatment plant except in times of heavy rainfall.

This scheme will provide a large-scale prototype of possible sewerage treatment schemes that can be used throughout western Queensland and other locations where there is plenty of land and where the local government doesn’t want to pay for installing and operating high cost solutions.

Response of Vetiver Grass to Extreme Nitrogen and Phosphorus Supply

Stefanie Wagner1, Paul Truong2, Alison Vieritz3 and Cameron Smeal4

1Faculty for Geosciences University of Hamburg, Germany2 Veticon Consulting, Brisbane, Queensland, Australia

3 Department of Natural Resources and Mines, Queensland, Australia4Davis Gelatine, Beaudesert, Queensland, Australia

Abstract: Due to its unique morphological and physiological characteristics, vetiver grass has a very high level of tolerance to both adverse climatic and edaphic conditions, and recently it has been found to have a very fast and high capacity for N and P uptake in wastewater and polluted water.

As a part of a project conducted to calibrate vetiver for use in a computer model - MEDLI (Model for Effluent Disposal Using Land Irrigation), a pot experiment was carried out to determine vetiver maximum capacity for N and P up take in soil with very high levels of these two elements, N supply up to 10 000 kg/ha/year and P at 1 000 kg/ha/year were used.

Results show that vetiver grass has a very high capacity of absorbing N at elevated levels of N supply. Vetiver growth will respond positively to N supply up to 6 000 kg/ha/year and supply as high as 10 000kg/ha of N did not adversely affect vetiver growth. These features make vetiver highly suitable for treating wastewater and other waste materials high in N.

Vetiver requirement for P was not as high as for N, and no growth response occurred at level higher than 250 kg/ha/year, but its growth was not adversely affected at the P application level as high as 1 000 kg/ha/year. However because of its very fast growth and high yield, the total amount of P uptake by vetiver still far exceeds those of other tropical and subtropical grasses.

Annex 2: Upflow Anaerobic Sludge Blanket (UASB) Reactors

Table A: Anaerobic Municipal Wastewater Treatment Plants Designed and Built by EarthCAD/IRAMconsult

Capacity (MLD) Design Population Location

200 1,000,000 Kanpur, Uttar Pradesh

152 760,000 Ludhiana, Punjab


60 300,000 Ujjain, Madhya Pradesh

60 300,000 Ras al Khaimah, UAE

50 250,000 Faridabad, Haryana


40 200,000 Karnal, Haryana


36 (mixed municipal & tannery wastewater) Kanpur, Uttar Pradesh

35 175,000 Panip, Haryana

30 150,000 Gurgaon, Haryana

30 150,000 Sonip, Haryana

30 150,000 Rajahmundry, Andhra Pradesh

25 125,000 Yamunanagar, Haryana

22 110,000 Nashik, Maharashtra


14 70,000 Mirzapur, Uttar Pradesh

10 50,000 Panip, Haryana

10 50,000 Yamunanagar, Haryana

8 40,000 Chapra, Bihar

5 25,000 Kanpur, Uttar Pradesh

Totals

999 4,965,000

Technical Description and Design Parameters of UASB Reactors

sludge blanket

settling zone

excess sludge

effluent

pre-treated wastewaterbiogas bubble

fine sludge

heavy sludge

biogas

deflector

Figure A is a schematic diagram

illustrating the main elements of an upflow anaerobic sludge blanket (UASB) reactor: the influent distribution system, the sludge blanket, the gas dome and the effluent collection gutters.1

Figure A. Schematic Diagram of a UASB Reactor

Performance:

Removal efficiencies depend on water temperature, hydraulic loading, reactor design, the quality of the works and the maintenance status of the reactor. Depending on the composition of the wastewater, the removal efficiency of the UASB process may vary between 60-70 percent for COD and 75-85 percent for BOD5 at influent temperatures between 20-35o. At 24o C a properly designed and built reactor treating a typical municipal wastewater, when operated within design parameters, may be expected to average removal efficiencies of 75 percent of BOD, 70 percent of COD and 80 percent of TSS. Only negligible amounts of nitrogen and phosphorus are removed; 75-90 percent of N will be converted to ammonium ion (NH4

+). Sulfur compounds are almost completely converted to hydrogen sulfide (H2S). Removal of low concentrations of helminth ova is almost complete, but in endemic regions with high concentrations, 80-90 percent removal may be expected. Removal of pathogenic bacteria and viruses is about 50 percent.

The composition of biogas generated in the reactor depends on the characteristics of the wastewater and on the loadings applied. Gas production is typically 220-250 l/kg of influent COD, excluding gas dissolved in the effluent. For an influent COD concentration of 300 mg/l, gas production will be about 60-75 l/m3 of treated wastewater. The measured gas production is the primary control parameter of the reactor, e.g., the parameter that indicates whether the reactor is functioning properly. Lower production indicates inhibition of the biological process, sludge loss or some other problem. Sludge production depends mainly on the concentration and organic content of suspended solids in the wastewater and the SRT and is adversely affected by sludge washout.

1 Source: Haskoning Consulting Engineers and Architects; excerpted from their design manual for UASB reactors.

Annex 3: Constructed Wetlands Using Vetiver Grass as a Biological Filter

Source: EarthCAD, Inc.

Diagram of a constructed wetland using vetiver grassto polish anaerobically pretreated wastewater

effluents

grade level

Roots are harvested to maintain sufficient

hydraulic flow area, but less often than roots (1-2

years), and vetiver hedges are re-

established by dividing the root crown

into “slips” that are “replanted”

Regeneration of vetiver biomass

maintains a dynamic sink for mineralsand carbon

The fine, extensive vetiver root system absorbs mineralized nutrients and salts from the effluent

The large surface area of the vetiver

root mass hosts films of heterotrophic

bacteria that decompose residual COD and attenuate refractory organics

Effluent from initial

treatment in a UASB

anaerobic reactor

The vetiver “biofilter” may be sized to achieve

the desired effluent quality up to tertiary

MaturehedgeFloats support a

hedge of vetiver grass above an effluent stream,

and the roots hang free in the water

column

1.0 m

On a 2-3 month cycle the leaves of mature hedges are harvested, leaving

the roots intact

Root crown water level

baffle

Vetiver roots actively oxygenate

the water column by means of its

aerenchyma tissue, a charcteristic of aquatic plants

The final effluent from a vetiver wetland will

contain no algae, unlike waste

stabilization pond effluent

Low Cost WW Treatment

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