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Technology Selection Reflections. Getting rid of all the muck Biggest bang for the buck Reliability, no need for luck. Selecting a Treatment Process. Input. Algorithm. Output. Water characteristics. Treatment Process. Resources (Capacities). Decision. Institutional. Economic.

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Technology selection reflections

Technology Selection Reflections

Getting rid of all the muck

Biggest bang for the buck

Reliability, no need for luck

Selecting a treatment process
Selecting a Treatment Process




Water characteristics



Resources (Capacities)




Labor force




Decision quality as f data quantity
Decision Quality as f(Data Quantity)


Treatment Choice

Decision Quality

More data, but no design change!

Amount of Data

Better default!

How could you increase the y intercept? ____________

Identify critical data!

How could you increase the slope? _________________

Optimal water treatment decision
Optimal Water Treatment Decision

  • Sustainable

  • Improvement in

    • Public health (risk reduction)

    • Labor savings

    • Individual and community empowerment

  • At a cost/benefit ratio that is commensurate with competing expenditures and interventions

An optimization problem with many options
An Optimization Problem with Many Options

  • Technology

    • Water sources

    • Water treatment processes

    • Water storage

    • Water distribution

    • Separate drinking water from other uses (bottled water)

  • Scale (household to municipal)

  • Staging (order of implementation)

    • Sustainable Staged Space

Data quality
Data Quality

  • Many of the choices are discrete (either process A or B or C)

  • Thus there are regions with additional data that don’t cause any improvement in design

  • How can we choose which data to gather to maximize the rate of approach to the optimal design?

We will return to this question after we review our options

What are our choices clean water combos
What are our Choices?Clean Water Combos


  • Water Source

    • Scale, type, characteristics

  • Treatment

    • Scale, capacity, processes, automation

  • Storage

    • Scale, capacity

  • Distribution resolution

    • Scale, capacity


Water characteristics source
Water Characteristics: Source

  • Rain

    • Treat as if it were surface water

  • Groundwater

    • If “under the influence,” then treat as if it were surface water

  • Surface

  • Ocean

Water treatment objectives

Particle removal

Get turbidity below

30 NTU (WHO limit for disinfection only treatments)

5 NTU (Particle removal technologies should exceed this goal)

Pathogen inactivation/removal

Hazardous chemical removal

Naturally occurring




Anthropogenic contamination

Water Treatment Objectives

Microbiological Safety

Chemical Safety


WHO is working on guidance for these contaminants


Particle removal big scale






Particle Removal: Big Scale





Operator Skill*




*EPA’s opinion, not WHO’s opinion!

Approximate turbidity range

Particle removal small scale






Particle Removal: Small Scale
















Who on particle removal for pou
WHO on Particle Removal for POU

  • There is a need to investigate, characterize and implement physical and physical-chemical technologies for practical and low cost pre-treatment

  • Some physical or physical-chemical methods may be highly effective for treatment of stored household water on their own. (i.e., won’t need disinfection)

  • Particle removal technologies include:

    • Settling or plain sedimentation

    • Fiber, cloth or membrane filters

    • Granular media filters

    • Slow sand filter

Who on ssf as pou

  • Slow sand filtration is the least likely to be sustainable at the household level.

    • the preferred filter designs and installations often are larger and capable of treating more water than needed by individual households

    • because of their relatively large size (surface area)

    • and the needs for

      • proper construction and operation,

      • regular maintenance (especially sand scraping, replacement and cleaning) by trained individuals.

  • Such demands for achieving good performance are unrealistic because they are beyond the capacities and capabilities of most households

Need a good small-scale design!

Need a simple cleaning technique!

What was who thinking about ssf
What was WHO thinking about SSF?

  • How much water will this system produce?

    • _____ m/hr

    • _____ m/d

    • _____ m3

  • Why won’t this system work well?

0.45 m




Ssf design flaws
SSF Design Flaws…

Flow control (“floating weir”)

Can’t handle much head loss

Scour when head loss is low

Requires a hill side

Siphon risk-

Top layer of sand can dewater if supply water stops or if head loss is low

3 200 L drums


Takes up lots of space

Flow control failure
Flow Control Failure

  • A floating weir (that can be made of a bowl, two small tubes and a hose) in the supply tank maintains a constant flow of water to the top of the filter tank

  • Environmental Health Project (WASH ) concludes that the close attention and frequent adjustment required to operate demonstration models has resulted in early abandonment

Why doesn t this work well

Where is constant head?

Where is head loss element?

How is flow adjusted?

What is the role of the nylon string?

What happens when you add a pebble?

How flexible is a rubber tube?

Why doesn’t this work well?

The proctor and gamble solution pur
The Proctor and Gamble Solution: PuR

  • The PuR product uses ferric sulfate, bentonite, sodium carbonate, chitosan, polyacrylamide, potassium permanganate, and calcium hypochlorite

  • A small sachet of powdered product visibly separates the cleaned water from the murky masses

  • Initial efforts are underway to develop a sustainable market-based approach for delivery and to learn how to best make POU products available. Three separate complementary models are being explored:

    • a social model led by non-profit organizations

    • a commercial model led by the private sector

    • an emergency relief model led by relief organization

  • One small sachet, costing about US $0.10 in the commercial model, will treat 10 liters of water (enough drinking water for an average family for two days)

Pur directions
PuR: Directions

  • Add 1 sachet to 10 litres of water and stir to begin process of separating the cleaned water from the murky masses

  • Stir water for 5 minutes until clear

  • Filter water through a cloth and dispose of separated floc in the latrine

  • Let clear water stand for 20 minutes to allow for complete disinfection

  • Store in a suitable container to prevent recontamination

No sedimentation?

Pur microorganisms and arsenic removal
PuR: Microorganisms andArsenic Removal

  • PuR is expected to provide excellent disinfection (>7-log bacterial, >4-log viral and >3-log parasite reductions) across a variety of water types and under conditions that stress less effective purification products including solar or chlorine treatment alone

  • No E. coli were detected post-treatment in any of 320 samples of drinking water sources collected in developing countries

  • The POU treatment was also effective in removing arsenic from water artificially contaminated with arsenic and from water with naturally occurring arsenic contamination

  • In Bangladesh tests, arsenic decreased by a mean of (85%) 88% of treated samples were <50 ppb

Pur turbidity range
PuR Turbidity Range

  • Turbidities in the samples were reduced significantly, pre-treatment ranged from 0 to 1850 NTU (mean 19 NTU) and final values were generally less than 1 NTU (average 0.25 NTU).

  • The highest final turbidity observed was 3.2 NTU for a water source whose starting turbidity had 1850 NTU

Pur critique
PuR Critique

  • This is not sustainable or in the interests of people in rural areas.

  • It becomes a product that has to be purchased on a regular basis from a foreign country.

  • I think the analogy to the scandalous infant formula problems of a couple of decades ago should be kept in mind where people were encouraged to abandon breast feeding in favor of a foreign infant formula.

  • Getting people “hooked” on a product that will require as much as 10% of their income instead of trying to develop sustainable solutions that don’t have recurrent cost and that the villagers have control over is exploitive in the worst of ways

--Humphrey Blackburn*

*Okay, he designs and sells slow sand filters…

Particle removal small scale1






Particle Removal: Small Scale
















Minimal data requirements for surface water treatment
Minimal Data Requirements for Surface Water Treatment

  • What would you need to know before you would be willing to recommend a water treatment technology for a community of 250 that is currently relying on an untreated surface water source?

Minimal data
Minimal Data…

Will determine treatment technology

  • Turbidity

  • Pathogens

  • Chemicals

    • Determine if naturally occurring contaminants are present in region

    • Assess watershed exposure risk to agricultural and industrial contamination

  • Economic, Institutional, Educational Capacity

Assume pathogens are present!

The choice of scale



The Choice of Scale

  • My long held assumption that only centralized systems made sense

  • Remember creativity: vary parameters over the full range of possibilities

    • Vary number of customers per treatment plant!

  • Are there situations where decentralized is better?

Centralized models in the global north
Centralized Models in the Global North

  • Centralized (Municipal)

    • Water source (possibly multiple sources)

    • Treatment (possibly multiple facilities)

    • Storage (usually multiple tanks in sprawling communities)

    • Distribution (one network with redundancy)

  • Governance

    • Federal or State regulations

    • City department, Commission

  • Ownership

    • Private or Public

Decentralized models in the global north
Decentralized Models in the Global North

  • Single source, treated as needed, stored (often in a pressure tank in the basement)

  • Owned and maintained by the homeowner

  • Initial local health department inspection

  • Additional testing at homeowner’s initiative

  • Example… Household wells

Epa s case for pou poe
EPA’s case for POU/POE

  • Public water supply consumers may not always possess the financial resources, technical ability, or physical space to own and operate custom-built treatment plants

  • Small drinking water treatment systems, such as Point-Of-Use and Point-Of-Entry (POU/POE) units, may be the best solution for providing safe drinking water to individual homes, businesses, apartment buildings, and even small towns

  • These small system alternatives can be used for not only treating some raw water problems, but they are excellent for treating finished water that may have degraded in distribution or storage or to ensure that susceptible consumers, such as the very young, very old, or immuno-compromised, receive safe drinking water

Pou poe concerns
POU/POE Concerns

  • The problem of monitoring treatment performance so that it is comparable to central treatment

  • POU devices only treat water at an individual tap (usually the kitchen faucet) and therefore raise the possibility of potential exposure at other faucets. Also, they do not treat contaminants introduced by the shower (breathing) and skin contact (bathing)

  • These devices are generally not affordable by large metropolitan water systems

  • POU devices are only considered acceptable for use as interim measures, such as a condition of obtaining a variance or exemption to avoid unreasonable risks to health before full compliance can be achieved

Poe solutions
POE Solutions

  • The 1996 regulations required the POU/POE units to be

    • owned, controlled, and maintained by the PWS or by a person under contract with the PWS operator to ensure

      • proper operation and maintenance

      • compliance with the MCLs or treatment technique

    • equipped with mechanical warnings to ensure that customers are automatically notified of operational problems

  • Under this rule, POE devices are considered an acceptable means of compliance because POE can provide water that meets MCLs at all points in the home

Could each community in the Global South have a designated person who maintains the POU devices?

Pou wins over centralized treatment when
POU wins over Centralized Treatment when…

  • The distance between houses is large (order 1 km) then POU supplies are common

  • The centralized system is unreliable (low institutional capacity, poor infrastructure)

  • The cost of POU treatment is less than the cost of a centralized treatment facility (small communities)

  • POU only treats water for human consumption (with savings in capital, operation, and maintenance costs)

Opening question
Opening Question

  • You live in a small community that chlorinates a surface water with turbidities that range between 5 and occasionally 200 NTU

  • Give 2 reasons why a POU SSF might not be a good solution

  • What research would you like to conduct to determine how serious these problems are?

Reactor challenges for pou
Reactor Challenges for POU

  • Flow rate control

  • Batch vs. continuous flow

  • Quantity of water to treat

  • Operation and Maintenance

  • Monitoring (or the lack thereof)

    • is there any indication of whether the POU device is working?

  • Failure modes… HACCP

Water safety plan
Water Safety Plan

  • Risk assessment to define potential health outcomes of water supply

  • System assessment to determine the ability of the water supply system to remove pathogens and achieve defined water quality targets (remember the chlorinator assignment?)

  • Process control using HACCP

  • Process/system documentation for both steady state and incident-based (e.g., failure or fault event) management

Hazard analysis at critical control points haccp
Hazard Analysis at Critical Control Points (HACCP)

  • It is recommended that HACCP for household water collection, treatment and storage be applied in the context of a Water Safety Plan that addresses source water quality, water collection, water treatment, water storage and water use.


  • We need better solutions for

    • Particle removal

    • Chemical removal

  • Existing designs are too expensive, don’t work well enough, or require advanced operator skills

  • We need easy to use and cheap monitoring devices

  • Remove particles before disinfection (unless you are using heat)

  • Can we outperform PuR?

  • We need better guidance for technology selection based on turbidity (or other easily monitored parameters)

Two meanings!

Monitoring capabilities
Monitoring Capabilities

  • Chlorine disinfection – measure residual

    • Hach $0.27 to $1.25 per test

  • Too expensive for POU applications

  • Reasonable for community systems

Monitoring capabilities coliform
Monitoring Capabilities: Coliform

  • Current cost is several dollars per sample for membrane filtration (enumeration)

  • Absolutely prohibitive for POU monitoring

  • Difficult for small communities

  • MIT Design that matters is exploring cheaper methods of measuring coliform concentrations

    • Melted wax incubator

    • More economical filtration apparatus

  • Coliform removal is still one of the best ways to evaluate filter performance (remember bacteria are hard to remove)

Testing for coliform bacteria presence absence tests
Testing for Coliform Bacteria:Presence/Absence Tests

  • Colisureallows testing for coliform bacteria and/or E. coli in 24 - 28 hours.

  • The detection limit of ColiSure is 1 colony forming unit (CFU) of coliform bacteria or E. coli per 100 mL of medium.

  • If coliform bacteria are present, the medium changes color from yellow to a distinct red or magenta.

  • If E. coli are present, the medium will emit a bright blue fluorescence when subjected to a long wave (366 nm) ultraviolet (UV) light.

Testing for coliform bacteria membrane filtration
Testing for Coliform Bacteria: Membrane Filtration

  • Membrane filter

    • 0.45 μm pores

    • 47 mm in diameter

  • Filter 100 mL of water to be tested through the membrane filter

Membrane filtration
Membrane Filtration

Add 2 mL of m-endo broth (selective media)

Place membrane filter in the petri dish on top of the nutrient pad

Petri dish with sterile absorbent nutrient pad

Membrane filtration incubation and results
Membrane Filtration:Incubation and Results

  • Incubate for 24 hours at 35°C

  • Coliform bacteria grow into colonies with a green metallic sheen

  • Non-coliform bacteria may grow into red colonies

  • Coliform concentration is __________________









8 coliform/100 mL

Monitoring turbidity
Monitoring: Turbidity

  • Hach portable Turbidimeter: $837.00

  • Sechi disk (great for lakes…)

  • SODIS technique

Turbidity measurements

10° detector


Turbidity Sensors (approximate turbidity measurement)

sample cell

Turbidity Measurements


90° detector


0° detector

sample cell

Cheap turbidity measurements
Cheap Turbidity Measurements


  • What is our cheap detector?

  • What is the detector measuring?

  • How could you make a cheap method of measuring turbidity



Technology selection reflections