Disinfectionof Wastewater Effluent

AReview of Current Techniques

ReportNo WSAA 101

November 1996

SYNOPSIS

Disinfectionis the process of reduction of pathogens of concern to humans, animals, orplants to acceptable levels of risks of transmission of disease. This reportcritically examines the processes available for disinfection ofpathogen-containing wastewater and comparing their effectiveness, practicality,reliability, effects on the environment and human health and the costsinvolved. It also details the circumstances under which disinfection should bepractised. The disinfection methods generally considered for use in Australiaconsist of chemical methods (chlorine, chlorine dioxide and ozone), physicalmethods (UV irradiation and membrane microfiltration), and biological methods(ponds).

Whendetermining disinfection needs for wastewater treatment plant effluent, it isthe pathogens present in the effluent that need to be inactivated. Care shouldbe taken in using bacterial indicators such as E. coli or thermotolerant coliforms to design and controldisinfection processes as these indicator organisms are known to be moresusceptible to some disinfection processes than are pathogens. Theeffectiveness of disinfection methods should not relate to removal of indicatororganisms alone. Research has shown that there are large variations in pathogenremoval through processes in wastewater treatment plants. Literature valuesshould not be used to assess pathogen removal. Site specific data should alwaysbe obtained.

Tooptimise disinfection, it is advisable to reduce suspended solids levels to themost practicably achievable levels for the system, to reduce organic compoundsentering the wastewater treatment plant as these are difficult to remove in secondarytreatment plants without advanced treatment and to reduce inorganic compoundsthat interfere with the disinfection method.

Inrelation to specific disinfection methods a number of conclusions have beenreached. Briefly the effectiveness of chlorine for inactivation of viruses,helminths and protozoa is lower than for bacteria and depends to a large extenton having the appropriate conditions, viz. optimum pH, adequate chlorinecontact time and low levels of ammonia and suspended solids. Ultraviolet irradiationwhich is gaining popularity in Australia is effective for disinfection ofbacteria and viruses, but has yet to be fully assessed for inactivation ofprotozoa and helminths.

Alsoproblems exist when treating effluent with suspended solids greater than 20mg/L. Reliability of UV equipment was raised as a major concern by authoritiesconsidering UV irradiation as a disinfection method. Although very effective,chlorine dioxide has only been sparingly used in Australia because of itscomplexity which requires constant supervision and high operating costs. Theuse of ozone for disinfection of wastewater has not been practised in Australiato date. Membrane microfiltration is gaining popularity in Australia althoughit is relatively complex, requiring a high degree of maintenance and systemcontrol to provide continuous disinfection. Ponds have been traditionally usedin Australia and remain the disinfection method of choice by some authorities,however they have not been purposely designed for disinfection.

Copiesof the Report are available from WSAA, price $A60. Orders may be placed throughthe Bookshop at www.wsaa.asn.au or by email to info@wsaa.asn.au.