ČSN ISO 16075-5 - Směrnice pro využití čištěných odpadních vod pro projekty závlah - Část 5: Dezinfekce čištěných odpadních vod a ekvivalentní úpravy

3.1.11 UV transmittance

fraction of photons in the UV spectrum transmitted through a material such as water or quartz

Note 1 to entry: It is preferable that an online UVT sensor be installed and used to verify UVT.

Note 2 to entry: The wavelength of the UVT (%) should be specified, often using a path length of 1 cm. The measurement is calibrated compared to ultra pure water (ISO 3696 grade 1 or equivalent).

Note 3 to entry: UVT is related to the UV absorbance (A) by the following formula (for a 1 cm path length): % UVT = 100 × 10-A.

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Abbreviated terms

A254

absorbance at 254

CT

product of the total residual chlorine and contact time

DBP

disinfection by-products

EPA

Environmental protection agency

DOC

dissolved organic carbon

DVGM

German Technical and Scientific Association for Gas and Water (deutsher veriein des gas-und wasserfaches e.v.)

LP

low pressure

LPHO

low pressure high output

LRV

log removal value

MF

microfiltration

MP

medium pressure

MWCO

molecular weight cut off

NOM

natural organic matter

ONORM

Austrian Standard (Österreichisches Normungsinstitut)

QA/QC

quality assurance/quality control

RED

reduction equivalent dose

RO

reverse osmosis

TDS

total dissolved solids

THM

trihalomethanes

TMP

trans membrane pressure

TOC

total organic carbon

TWW

treated wastewater

UF

ultra-filtration

UV

ultraviolet

UVT

ultraviolet transmittance

WW

wastewater

Wastewater pathogenic contaminants and their inactivation or removal

General

The most critical objective in a TWW reuse programme should be public health.

To achieve the main objective, other equally important objectives should be considered, including:

— environmental protection,

— aesthetics (odour and colour); and

— ability to meet irrigation requirements.

To protect public health and prevent environmental degradation, the TWW quality characteristics and pathogenic microorganisms contained in the wastewater should be assessed and consideration given to appropriate treatment to reduce the risk of negative impacts.

There are a wide range of technology options available to meet the water quality goals and to reduce the risk of disease transmission from pathogenic microorganisms that can be present in TWW and to meet the water quality goals.

In regular wastewater treatment plants, the two main processes that reduce the concentrations of pathogenic microorganisms in the water should be:

— the wastewater treatment process itself, which is intended mainly to reduce concentrations of suspended and dissolved organic matter;

— the process of disinfection of the TWW.

Type and occurrence of pathogens in wastewater

Urban wastewater intended for agricultural irrigation or for other purposes contains a variety of pathogenic microbial contaminants that can pose a risk to public health.

The type and number of pathogenic microorganisms in urban wastewater varies between countries and cities and with time/season (wet and dry), epidemics etc. When selecting disinfection method(s) the range of microorganisms that can be present should be considered, including parasites eggs, bacteria, amoebas and other protozoa, Giardia and viruses. Common infectious agents, associated diseases, and potential numbers of microorganisms found in domestic wastewater are shown in Table 1[2] (for the complete table see Table A.1).

Table 1 — Infectious agents potentially present in untreated (raw) wastewater[2]

Pathogen

Disease

Numbers in raw wastewater (per litre)

Shigella`

Shigellosis (bacillary dysentery)

Up to 104

Salmonella

Salmonellosis, gastroenteritis (diarrhoea, vomiting, fever), reactive arthritis, typhoid fever

Up to 105

Vibro cholera

Cholera

Up to 105

Campylobacter

Gastroenteritis, reactive arthritis, Guillain-Barré syndrome

Up to 104

Enteroviruses (polio, echo, coxsackie, new enteroviruses, serotype 68 to 71)

Gastroenteritis, heart anomalies, meningitis, respiratory illness, nervous disorders, others

Up to 106

Adenovirus

Respiratory disease, eye infections, gastroenteritis (serotype 40 and 41)

Up to 106

Rotavirus

Gastroenteritis

Up to 105

Entamoeba

Amebiasis (amebic dysentery)

Up to 102

Giardia

Giardiasis (gastroenteritis)

Up to 105

Cryptosporidium

Cryptosporidiosis, diarrhoea, fever

Up to 104

Ascaris

Ascariasis (roundworm infection)

Up to 103

Ancylostoma

Ancylostomiasis (hookworm infection)

Up to 103

Trichuris

Trichuriasis (whipworm infection)

Up to 102

The practical measurement of all pathogenic pollutants in TWW is almost impossible.

The main reasons are:

— low concentrations of the pathogenic contaminants in the TWW;

— limitation of present technology, to detect pathogens when they are present in low numbers;

— testing for pathogenic contaminants in the laboratory is lengthy and expensive.

Consequently, the control and monitoring of pathogenic microorganisms should be done by testing for indicator microorganisms, which are feasible and simple to measure as a result of their much larger numbers, and based on the premise that factors and treatment affecting their removal similarly affect the pathogens of interest.