Wastewa­ter Treat­ment through Chem­ical Pro­cesses

Neut­ral­iz­a­tion

Wastewa­ter tech­no­logy uses neut­ral­iz­a­tion to ad­just the pH value. Acids or al­kali are ad­ded, as re­quired, after pro­cesses like pre­cip­it­a­tion and floc­cu­la­tion, and for the neut­ral­iz­a­tion of in­dus­trial wastewa­ters.

Ox­id­a­tion/​Re­duc­tion

Redox re­ac­tions are fre­quently util­ized in chem­ical wastewa­ter treat­ment and in the treat­ment of pot­able wa­ter. Ox­id­a­tion pro­cesses with ozone and hy­dro­gen per­ox­ide ef­fi­ciently re­move chlor­in­ated hy­dro­car­bons and pesti­cides from pot­able wa­ter.  

In wastewa­ter treat­ment, ox­id­a­tion pro­cesses are used to re­move dif­fi­cult bio­de­grad­able com­pounds. Par­tic­u­larly ef­fi­cient is pho­to­chem­ical puri­fic­a­tion, which forms hy­droxyl rad­ic­als from hy­dro­gen per­ox­ide or ozone through UV-light ex­pos­ure. These Ad­vanced Ox­id­a­tion Pro­cesses (AOP) are used to de­grade drug sub­stances like an­ti­bi­ot­ics, cyto­static drugs, hor­mones and other an­thro­po­genic trace sub­stances.  

In ad­di­tion, ozone aids in the ox­id­iz­a­tion of iron and man­ganese in well wa­ter. Re­duc­tion pro­cesses are re­quired to trans­form heavy metal ions, for in­stance, into eas­ily dis­sol­uble sulf­ides.

Ad­sorp­tion and Chemisorp­tion

Ad­sorp­tion is the ac­cu­mu­la­tion of sub­stances on the sur­face of a solid body, which is a phys­ical pro­cess where mo­lecules stick to bound­ary sur­faces through the van der Waal force. If chem­ical bond­ing binds sub­stances to the sur­face of a solid body, the pro­cess is called chemisorp­tion. In con­trast to ad­sorp­tion, chemisorp­tion is non-re­vers­ible.  

Wastewa­ter treat­ment uses ac­tiv­ated car­bons to bind sol­uble wa­ter con­tents that could not be suf­fi­ciently re­moved with lower-priced meth­ods such as bio­lo­gical wastewa­ter treat­ment, pre­cip­it­a­tion and floc­cu­la­tion. Col­or­ants from tex­tile dy­ing plants, for in­stance, of­ten can only be com­pletely re­moved through ad­sorp­tion on ac­tiv­ated car­bon. An­thro­po­genic trace ele­ments such as phar­ma­ceut­ical residues and po­lar or­ganic sub­stances like ad­sorb­able, or­gan­ic­ally-bound halo­gens (AOX) also bind to ac­tiv­ated car­bon.  

Doped ac­tiv­ated car­bon can also be em­ployed to re­move ar­senic and heavy metals. Gran­u­lated iron hy­drox­ide is an­other ideal agent to re­move toxic metal­loid ar­senic from pot­able wa­ter, con­tam­in­ated ground wa­ter and in­dus­trial wastewa­ters. In this pro­cess, the iron hy­drox­ide re­acts with the ar­sen­ate ions to form iron ar­sen­ate. This method is ef­fi­cient as well as cost-ef­fect­ive.

Pre­cip­it­a­tion

Pre­cip­it­a­tion is a chem­ical pro­cess that sep­ar­ates a pre­vi­ously sol­uble sub­stance from a fluid. A com­mon method is to cre­ate a pre­cip­it­a­tion re­ac­tion by adding suit­able agents. Through pre­cip­it­a­tion, heavy metals, for in­stance, trans­form to not eas­ily sol­uble metal hy­drox­ides. Other situ­ations may re­quire pre­cip­it­a­tion to car­bon­ates or sulf­ides.  

An­ions can of­ten be pre­cip­it­ated as cal­cium, iron, and alu­minum salts. The sep­ar­a­tion of flu­or­ide ions, for in­stance, is achieved through pre­cip­it­a­tion with milk of lime. Dur­ing wastewa­ter treat­ment in the treat­ment plant, adding salts like iron(II) sulfate, iron chlor­ide or alu­minum chlor­ide lowers the phos­phate con­cen­tra­tion. The phos­phate pre­cip­it­a­tion can either be in­teg­rated as sim­ul­tan­eous pre­cip­it­a­tion into the bio­lo­gical treat­ment stage or ad­ded as a sub­sequent sep­ar­ate pro­cess step.

Floc­cu­la­tion

Floc­cu­la­tion pre­pares very fine particles that are present either sus­pen­ded or in the form of col­loidal solu­tions, for re­moval from wa­ter. If the sur­face charge of this very fine par­tic­u­late mat­ter is the same, the particles can­not, due to mu­tual elec­trical re­pul­sion, ac­cu­mu­late to lar­ger ag­glom­er­ates.

In this case, suit­able chem­ic­als, floc­cu­lants and floc­cu­la­tion aids help achieve the ag­glom­er­a­tion of such par­tic­u­late mat­ter, cre­at­ing macro flakes that sed­i­ment. Floc­cu­la­tion is used to im­prove set­tling prop­er­ties as well as to drain sewage sludge. Em­ploy­ing iron and alu­minum salts for floc­cu­la­tion al­lows the floc­cu­lat­ing of phos­phate at the same time.

Ion Ex­changer

Ion ex­changers are ma­ter­i­als that can re­place the ions of one solu­tion with other ions. The cation ex­changer, for in­stance, re­places cal­cium ions with so­dium ions. Once the ion ex­change is ex­hausted and the cal­cium ions are com­pletely sat­ur­ated, the ion ex­changer needs re­gen­er­a­tion.  

This suc­cess of this pro­cess is based on the prin­ciple of dis­place­ment: the higher the ions’ charge, the stronger the ion-bind­ing to the ion ex­changer. If both types of ions are charged the same, the one with the lar­ger ra­dius will be the one with the stronger ion-bind­ing force. Dur­ing the ion ex­change pro­cess, the stronger-bind­ing ion will dis­place the lesser-bind­ing ion.

Ion ex­changers are suit­able for the re­moval of heavy metals and an­ions and are there­fore of­ten used as ‘poli­cing fil­ters’ after pre­cip­it­a­tion and floc­cu­la­tion. In ad­di­tion, they as­sist with wa­ter soften­ing, chan­ging the wa­ter’s salt con­tent, and wa­ter de­sal­in­a­tion; par­tic­u­larly im­port­ant for the semi­con­ductor in­dustry that uses them to pro­duce ex­tremely clean, de­min­er­al­ized wa­ter known as ul­tra pure wa­ter.

*source: das-ee.com