In the mid-to-late 1800s, chlo­rine had been used spo­rad­i­cal­ly to help con­trol wa­ter­borne ill­ness­es and pol­lu­tants in drink­ing wa­ter. This al­so prompt­ed util­i­ties across the world to be­gin large-scale fil­tra­tion of wa­ter sup­plies and waste­water treat­ment. How­ev­er, back then fil­tra­tion alone was in­ad­e­quate to re­duce wide­spread ill­ness­es and dis­in­fec­tion with chlo­ri­na­tion vir­tu­al­ly elim­i­nat­ing wa­ter­borne epi­demics, in­creas­ing life ex­pectan­cy by 50 per cent.

Con­ven­tion­al wa­ter and waste­water treat­ment fa­cil­i­ties use var­i­ous dis­in­fec­tion process­es at the head of the treat­ment work with the aim of not cre­at­ing chlo­rine byprod­ucts, there­after chlo­rine is ap­plied to the ef­flu­ent or fin­ished wa­ter prod­uct to main­tain a resid­ual through­out the wa­ter dis­tri­b­u­tion sys­tem. With in­creas­ing wa­ter pol­lu­tion and strin­gent en­vi­ron­men­tal reg­u­la­tions, it is be­com­ing more dif­fi­cult to achieve all of the re­quire­ments for safe Wa­ter and Waste­water dis­in­fec­tion with one treat­ment alone. To­day, a more lay­ered ap­proach for dis­in­fec­tion with mul­ti­ple tech­nolo­gies used to­geth­er is of­ten nec­es­sary. Some of the most re­li­able dis­in­fec­tants and dis­in­fec­tion process­es used in in­dus­try to­day in­clude:

(I) Chlo­rine gas

Used in wa­ter treat­ment plants since the ear­ly 1900s, it is es­sen­tial­ly pure chlo­rine, typ­i­cal­ly de­liv­ered in pres­surised cylin­ders. It is gen­er­al­ly the most cost-ef­fec­tive, ef­fi­cient, and eas­i­est to main­tain the method of dis­in­fect­ing with chlo­rine. Chlo­rine diox­ide is gen­er­at­ed by mix­ing acid or chlo­rine gas and sodi­um chlo­rite then mixed with ejec­tor wa­ter as an en­trained gas to form a so­lu­tion that is ap­plied to the process. Be­cause it can­not be com­pressed and liq­ue­fied for trans­porta­tion, it is of­ten gen­er­at­ed on-site close to its in­tend­ed use. It is a strong ox­i­dant and a very pop­u­lar dis­in­fec­tant across a wide pH range for both wa­ter and waste­water. It does not re­act with am­mo­nia to be­come a weak­er dis­in­fec­tant, im­por­tant for wa­ter and waste­water treat­ment plants where the wa­ter has a high am­mo­nia con­tent, of­ten re­sult­ing in low­er op­er­at­ing costs. Chlo­rine diox­ide is fre­quent­ly used in wa­ter treat­ment plants as a pri­ma­ry dis­in­fec­tant ear­ly in the treat­ment process to pre­vent the for­ma­tion of Tri­halomethanes (THMs) which en­vi­ron­men­tal pol­lu­tants, and many are con­sid­ered car­cino­genic.

(II) Sodi­um Hypochlo­rite

Man­u­fac­tured at ap­prox­i­mate­ly 12 to 15 per cent chlo­rine by weight with a pH greater than 11. De­liv­ery sys­tems of this liq­uid dis­in­fec­tion in­clude stor­age tanks, chem­i­cal dos­ing pump­ing sys­tems with as­so­ci­at­ed valves and pip­ing, and a con­trol method such as flow con­trol, resid­ual con­trol or com­pound loop (flow + resid­ual). Al­though more ex­pen­sive com­mer­cial­ly than chlo­rine gas it is con­sid­ered sim­pler to main­tain and op­er­ate with on-site gen­er­a­tion. Bulk sodi­um hypochlo­rite con­cen­tra­tion de­cays over time and high­er vol­umes are re­quired to achieve the same re­sult.

(III) On-site hypochlo­rite gen­er­a­tion (OS­HG)

An On-site hypochlo­rite gen­er­a­tion (OS­HG) sys­tem us­es elec­trol­y­sis to gen­er­ate a nom­i­nal so­lu­tion of hypochlo­rite on-site as need­ed. A brine so­lu­tion pass­es through an elec­trolyt­ic cell, con­vert­ing the chlo­ride ions from salt to hypochlo­rite. The process typ­i­cal­ly us­es three pounds of salt, two kW hours of elec­tric­i­ty and 15 gal­lons of wa­ter to pro­duce a pound of chlo­rine in 15 gal­lons of so­lu­tion, the equiv­a­lent of the ac­tive chlo­rine present in one gal­lon of 12.5 per cent bulk hypochlo­rite or one pound of chlo­rine gas. OS­HG sys­tems have mod­er­ate main­te­nance re­quire­ments and are con­sid­ered safer than trans­port­ing chlo­rine gas un­der pres­sure.

(IV) On-site chlo­rine gen­er­a­tion (OS­CG)

Sim­i­lar to OS­HG, an On-site chlo­rine gen­er­a­tion (OS­CG) sys­tem us­es brine so­lu­tions and mem­brane elec­trol­y­sis to pro­duce high­er con­cen­tra­tions of hypochlo­rite up to 12 per cent of chlo­rine gas. On-site chlo­rine gen­er­a­tion re­duces trans­porta­tion and de­liv­ery haz­ards as­so­ci­at­ed with ship­ping com­mer­cial hypochlo­rite and elim­i­nates trans­porta­tion safe­ty con­cerns of chlo­rine gas de­liv­ered in liq­uid (bulk) con­tain­ers.

(V) Cal­ci­um hypochlo­rite

A sol­id tablet, cal­ci­um hypochlo­rite is typ­i­cal­ly 60 per cent avail­able chlo­rine de­liv­ered via a di­lu­tion tank or tablet feed sys­tem where­by the cal­ci­um hypochlo­rite is dis­solved in­to so­lu­tion then dosed with a me­ter­ing pump. It is com­mon­ly used in swim­ming pools. Be­cause cal­ci­um hypochlo­rite is ex­pen­sive per pound of chlo­rine and is dif­fi­cult to ac­cu­rate­ly dose when it is oc­ca­sion­al­ly used for wa­ter and waste­water treat­ment, it is typ­i­cal for small­er re­mote plants or treat­ment works where oth­er meth­ods of chlo­rine feed are not fea­si­ble.

(VI) Chlo­ramines

In the pres­ence of am­mo­nia, chlo­rine com­bines with the am­mo­nia to form ei­ther mono-chlo­ramine, di-chlo­ramine or tri-chlo­ramine de­pend­ing on the am­mo­nia-to-chlo­rine ra­tio. Mono-chlo­ramine is a rel­a­tive­ly weak dis­in­fec­tant but main­tains a very sta­ble long-last­ing resid­ual in wa­ter and is there­fore of­ten used as a sec­ondary dis­in­fec­tion method in sys­tems with a long wa­ter age. Chlo­ramine sys­tems can be chal­leng­ing to con­trol and wa­ter and waste­water op­er­a­tors and en­gi­neers must care­ful­ly mon­i­tor and main­tain their dos­ing sys­tems for max­i­mum ac­cu­ra­cy.

(VII) Ozone

Com­priss­ing three oxy­gen atoms (O3), ozone is a very strong ox­i­dant. It de­te­ri­o­rates rapid­ly to oxy­gen and is usu­al­ly gen­er­at­ed on-site us­ing ei­ther air or pure oxy­gen. Ozone does not pro­duce dis­in­fec­tion byprod­ucts (DBPs) and can be used as a pri­ma­ry dis­in­fec­tant for wa­ter treat­ment to re­duce THMs and DBPs. Ozone al­so is used for taste, odour and colour con­trol in potable wa­ter treat­ment, as well as Iron-Man­ganese (Fe/Mn) re­moval when THMs are a con­cern. Ozone can al­so be used to re­move mi­cro-pol­lu­tants in­clud­ing pes­ti­cides at dis­in­fec­tion dosages. Ozona­tion is typ­i­cal­ly not used for pri­ma­ry dis­in­fec­tion of waste­water ef­flu­ent with high lev­els of sus­pend­ed solids (SS), bio­chem­i­cal oxy­gen de­mand (BOD), chem­i­cal oxy­gen de­mand (COD), or to­tal or­gan­ic car­bon (TOC) since the cost of treat­ment can be rel­a­tive­ly high in cap­i­tal and in pow­er in­ten­sive­ness.

(VI­II) Ul­tra­vi­o­let Light (UV ra­di­a­tion)

Ul­tra­vi­o­let light en­er­gy at 254 nm wave­length is ab­sorbed by the DNA of a mi­croor­gan­ism, stop­ping the re­pro­duc­tive process and ren­der­ing it non-in­fec­tive and mi­cro­bi­o­log­i­cal­ly dead. UV sys­tems op­er­ate at vary­ing pres­sure and out­put, de­pend­ing on ap­pli­ca­tion, tar­get pathogens and wa­ter qual­i­ty in­volved. UV has most re­cent­ly been used to treat waste­water ef­flu­ent since en­vi­ron­men­tal reg­u­la­tions glob­al­ly re­quire more strin­gent chlo­rine dis­charge lim­its for var­i­ous re­ceiv­ing streams. UV is ef­fec­tive in re­mov­ing chlo­rine-re­sis­tant pathogens from drink­ing wa­ter in­clud­ing Cryp­tosporid­i­um, Gi­a­r­dia and var­i­ous virus­es that have proven to be re­sis­tant to tra­di­tion­al dis­in­fec­tion meth­ods such as chlo­rine and fil­tra­tion.

(IX) Per­acetic Acid (PAA)

Per­acetic acid (CH3CO3—al­so known as per­ox­y­acetic acid, or PAA—is a liq­uid that func­tions as a strong ox­i­dis­ing agent, has an acrid odour and can al­so be used as a dis­in­fec­tant. PAA is gen­er­al­ly com­mer­cial­ly avail­able as an equi­lib­ri­um mix­ture of 12-15 per cent per­acetic acid and 18-23 per cent hy­dro­gen per­ox­ide. PAA is avail­able in gal­lon con­tain­ers and in bulk, re­quires stain­less steel pip­ing, and is ad­min­is­tered us­ing a me­ter­ing pump. Since PAA is a high­ly ef­fec­tive bac­te­ri­cide, does not form dis­in­fec­tion by-prod­ucts (DBPs), has min­i­mal de­pen­den­cy on pH and does not leave a resid­ual, it has re­ceived sig­nif­i­cant con­sid­er­a­tion for the dis­in­fec­tion of waste­water ef­flu­ent.

With a well-de­fined wa­ter and waste­water treat­ment process goals and ex­pe­ri­ence, knowl­edge­able part­ners work­ing to­geth­er, de­vel­op­ing the most ef­fec­tive, sim­ple and bud­get-con­scious wa­ter and waste­water dis­in­fec­tion plan should be a sim­ple, sus­tain­able and eco­nom­i­cal process.