Costing end point disinfection in water treatment plants

Budgets are tightening, and the responsibility to account fully for all input cost variables in water treatment systems is more important than ever. End-point disinfection offers a useful case study.

For example: everybody knows that bleach is cheap and chlorine gas is cheaper. But is it really? There is a long-held assumption that the cost of a chlorination programme can be determined by comparing the relative cost of the chemical (R/kg), but recent extensive studies have invalidated this perspective,” explains Peter Buchan, CEO of Control Chemicals.

In order to correctly calculate end-point disinfection, it is necessary to consider multiple components, including: • capital expenditure on the dosing equipment, as well as its depreciation and maintenance costs • the cost of ongoing training and labour needed to operate that equipment • energy/electricity consumption • risk/safety – insurance and potential injury costs • raw material/chemical used • performance. One complication arises from the fact that capital expenditure, operational expenditure, labour and training budgets typically fall into different departments or cost centres. “Many of these costs are not immediately obvious,” explains Peter Buchan, CEO of Control Chemicals.

“In order to fully assess the total cost of a disinfection technology or system, it can be illuminating to work backwards from the end result – meaning, once the mandated water quality has been reliably achieved, one should then cost out laterally from there. It is remarkable to find that efficacy and performance are often considered in isolation when disinfection systems are specified or budgeted for,” he says.

Fortunately, over the past few years, a series of proactive comparative case studies were conducted by a major local municipality, with the results shedding sufficient light on the

UV Chlorine Gas Sodium Hypochlorite Calcium Hypochlorite Maturation Ponds

Description Ultraviolet Radiation

•Generated by special lamps to disrupt organisms’ genetic material •Considered unsuitable for water containing high levels of suspended solids, turbidity, colour, or soluble organic matter

Liquefied Chlorine Gas (100%)

• Available in pressurised containers • Heavier than air – its high toxicity makes it extremely hazardous to produce, transport, store and use

Sodium Hypochlorite (Bleach)

• A low-stability liquid typically containing 5.0%-15% active ingredient • Very short shelf life and now proven to decay into toxic perchlorates unless used while fresh

Costs

Energy use Medium

Capex cost (civils / installation etc.)

High • Specialised equipment High • Specialised equipment • Mandated & rigid safety requirements Medium • Specialised equipment and protection against nature elements Low • Durable roto-moulded tablet dosing systems None

Chemical input cost (per kg)

Maintenance

None

High • UV lamp cleaning • Support, regular inspection, and maintenance of

UV systems High • Regular maintenance fittings and safety systems • Intensive monitoring and reporting-related costs • Ongoing specialised training required • System strip and cleaning Moderate • Regular maintenance of pumps, fittings, delivery plumbing • Pump calibration • Stock testing and rotation Low • Ensure roto-moulded generators are filled with tablets • Ensure water supply to dosing system Low • Ensure no difficulty with water overflow

Service

High • UV re-lamping High • Stripping of full system and replacement of parts Moderate • Full strip of system and replacement of parts Low • Quarterly manual cleaning of generator units (if required) High • 3- to 5-year pond sludge removal to landfill

Training Chemical Safety Storage / regulation

Worker risk exposure

Community / surrounding area risk exposure

Chemical stability

Ease of Use

System complexity

High

N/A

Low

Low

N/A

High • Intensive training required High • Intensive training required Medium • Staff training required to meet safety, technical and other requirements Low Low

Need for servicing and calibration

Need for operator attendance and monitoring

High • Regular inspection, specialised servicing and maintenance required High • Regular inspection, specialised servicing and maintenance required Medium • Regular inspection, specialised servicing and maintenance required Low • Quarterly assessment and cleaning of generator units (or as required) High • 3- to 5-year pond sludge removal to landfill

• Requires skilled operators • In-line energy monitoring • Remote monitoring and alarming recommended • Regular operator attendance required • Requires skilled operators • In-line residual chlorine monitoring • Remote monitoring and alarms a prerequisite • Requires skilled operators • In-line residual chlorine monitoring • Remote monitoring and alarms recommended • Requires semi- or lowskilled operators • In-line residual chlorine monitoring • Remote monitoring and alarming recommended N/A

Calcium Hypochlorite (Tableted/Pellets)

• Produced from chlorine gas and lime • Stable and generally safe to handle, transport and store

Maturation Ponds

• Also known as polishing ponds that generally follow either the primary or secondary pond/s • Primarily designed for tertiary treatment (removal of pathogens, nutrients and algae)

Low

Medium

High Low

Higher (in comparison with gas)

Moderate Low to zero

Higher (in comparison with gas)

Low None

None

Low

High risk

High (toxic gas) • PPE and training required High risk Low risk

Moderate • PPE required

Low Low risk

Low • Minimal PPE required

Low N/A

Low

Zero

Does not degrade/ decompose Degrades readily even under normal storage conditions Highly stable N/A

FIGURE 2 Opex costing results

subject for a detailed case study to be presented at the WISA conference in Cape Town in 2019.

Case study by a metro

The relative advantages and disadvantages of various common wastewater disinfection methods have been widely available via open-source resources for some time – e.g. the composite table on page 17 comparing chlorine gas, bleach, calcium hypochlorite, UV and maturation ponds.

In the referenced metro study, a similar but far more intensive ‘real world’ approach was taken to compare end-point disinfection between UV systems, chlorine gas, calcium hypochlorite tablets/chips, and natural bioremediation dams at 26 wastewater treatment works (WWTWs). In order to fully cost these systems, metrics such as electricity, chemical use, service and maintenance, and staff training were included, making this case study the most comprehensive of its kind in recent memory. Here follows selected excerpts from the WISA presentation:

CCTa is the actual flow, while CCtd is the design flow. CCTa is the flow used to conduct the study, while CCtd would be the results should the entire capacity of the WWTW have been used.

When analysing the operational expenditure results, it was found that UV disinfection was the most expensive, followed by gas, calcium hypochlorite (chips) and ponds, which were the least expensive.

As shown in Figure 1, the UV cost comprised electricity, maintenance and service expenditure. Gas comprised chemicals, maintenance, service and training. The chips were purely chemical costs and the ponds were maintenance and service.

The performance of each type of end-point disinfection was displayed in the bacteriological log reduction graph. The membrane (MBR) and active sludge process (ASP) will always be part of the wastewater treatment process Figure 2 displayed ponds as the least expensive form of end-point disinfection – it is also the least effective form of disinfection. This is because the is no residual disinfection in the water to inactivate bacteria. The ponds are used merely to separate solids. This is also true for UV disinfection: after the water passes through the light source, there is no residual disinfection in the water to inactivate bacteria that may survive or be introduced afterwards.

Calcium hypochlorite chips were proven to give the best log reduction. When there is a chlorine residual in wastewater, the oxidation cycle has been completed – bacteria is unlikely to be found in the water.

Conclusion

The case study highlighted that by merely pricing disinfection technologies by R/kg, ponds technology would be the best option, followed by chlorine gas, UV and then chips. However, there was a significantly different outcome when working out costing using R/ℓ and considering maintenance, electricity, training and servicing. Furthermore, these results indicate that municipalities would not only enjoy significant savings but also stimulate low- and semi-skilled job creation, and improved safety if multivariate costing models were applied.

CASE STUDIES

If you would like to find out more about the case studies, please contact Klorman Solutions at info@klorman.co.za.