Application of membrane processes for treatment and reuse of rendering plant wastewater

Abstract

Postrojenja za obradu životinjskih nusproizvoda (POŽN) ispunjavaju važnu ulogu u održivosti mesne industrije obradom nastalih nusproizvoda (opasnog otpada) te tako smanjuju negativan utjecaj mesne industrije na okoliš i ljudsko zdravlje. Tijekom obrade ovakve vrste opasnog otpada dolazi do potrošnje velike količine vode i proizvodnje veoma opterećene otpadne vode. Kako bi se riješili navedeni problemi, u ovom radu ispitana je primjena tlačnih membranskih postupaka ultrafiltracije (UF), nanofiltracije (NF) i reverzne osmoze (RO) za obradu i oporabu sekundarnog efluenta biološke obrade otpadne vode POŽN-a. POŽN zbog velike opterećenosti otpadne vode u sklopu postrojenja mora imati sustav za obradu otpadne vode. Otpadna voda POŽN-a ima povoljan omjer BPK/KPK što takvu vodu čini prikladnom za biološku obradu. Stoga se u ovom radu nije istraživala izravna obrada otpadne vode već njezin sekundarni efluent dobiven nakon biološke obrade. Ispitana je učinkovitost obrade primjenom komercijalnih ravnih UF membrana (MW, GM, CQ, GK, PT, PU i PW) i UF membrane u obliku šupljih vlakana (ZW-1) te ravnih NF/RO membrana (NF270, NF, NF90 i XLE). Za utvrđivanje učinkovitosti obrade praćeni su sljedeći parametri: električna provodnost (κ), mutnoća, sadržaj ugljika (ukupni ugljik – TC, anorganski ugljik – IC i otopljeni organski ugljik – DOC), kemijska potrošnja kisika (KPK), ukupni Kjeldahlov dušik (TKN), sadržaj aniona (Cl-, F-, NO2-, NO3-, PO43-, SO42- i Br-) i kationa (Na+, Ca2+, Mg2+, K+, NH4+ i Li+) i pH. UF membrane pokazale su sličan učinak separacije, a to uključuje učinkovito smanjenje mutnoće i KPK-a te uklanjanje DOC-a, ali loše zadržavanje dušika i soli. S druge strane, RO i NF membrane znatno su bolje zadržale dušik i otopljene soli, ali njihova učinkovitost je uvelike ovisila o primijenjenoj membrani. Radi usporedbe, čiste i korištene membrane okarakterizirane su infracrvenom spektroskopijom s Fourierovom transformacijom (FTIR) i skenirajućim elektronskim mikroskopom (SEM). Blokiranje membrana modelirano je primjenom modificiranih Hermijinih modela za četiri mehanizma blokiranja (potpuno blokiranje pora, djelomično blokiranje pora, unutarnje blokiranje pora i nastajanje kolača). Površine membrane okarakterizirane su goniometrom u svrhu određivanja hidrofilnosti membrana preko kontaktnog kuta vode. Ispitan je utjecaj predobrade sekundarnog efluenta nakon biološke obrade na blokiranje UF i NF membrana pješčanom filtracijom i kombinacijom koagulacije i pješčane filtracije te UF kao predobrade za NF i RO membrane. Pješčana filtracija nije se pokazala uspješnom u suzbijanju blokiranja UF i NF membrana. Koagulacija je optimirana s aspekta odabira najpogodnijeg koagulacijskog sredstva (FeCl3, Al2(SO4)3, Aquaklar A (10 % Al2O3) i Aquaklar C (18 % Al2O3)) i radnih uvjeta (pH vrijednost i koncentracija koagulacijskog sredstva). Pokazalo se da je najprikladnije koagulacijsko sredstvo FeCl3 pri pH vrijednosti 5,87 i koncentraciji 10 mg Fe3+ L^-1. Koagulacija pri optimalnim uvjetima i naknadna pješčana filtracija dokazale su se kao uspješna predobrada pri čemu je pad fluksa kod UF membrana smanjen za 50 – 95 %, a za NF membrane 76 – 95 %. S druge strane, primjenom UF s MW membranom kao predobradom za NF i RO smanjen je pad fluksa za 50 – 72 %, a predobradom sa ZW-1 membranom za 2 – 67 %. Ispitano je i kemijsko čišćenje UF membrana blokiranih tijekom obrade sekundarnog efluenta POŽN-a s dva sredstva (PermaClean 99 – alkalno sredstvo i PermaClean 77 – kiselo sredstvo). Pokazalo se da alkalno sredstvo znatno bolje uklanja blokirajuće tvari s površine membrane. Osim toga ispitan je utjecaj radnog tlaka prilikom UF sekundarnog efluenta na ireverzibilnost blokiranja te utjecaj temperature alkalnog kemijskog sredstva na učinkovitost čišćenja. U konačnici je predloženo idejno rješenje za oporabu sekundarnog efluenta POŽN membranskim postupcima, a ono uključuje predobradu koagulacijom pri utvrđenim optimalnim uvjetima, pješčanu filtraciju za uklanjanje zaostalih flokula, tri stupnja UF s MW membranom te završnu obradu UF permeata s XLE membranom. RO permeat ima potencijal za primjenu u proizvodnji pare, a RO retentat za održavanje higijene postrojenja i transportnih vozila. Primjenom predloženog rješenja moglo bi se oporabiti 47,3 % sekundarnog efluenta, 11,1 % kao kotlovska voda i 36,2 % za pranje podova.Rendering plants fulfill a key role in the sustainability of the meat industry by treating the resulting animal by-products – hazardous waste; thus, reducing the negative impact of the meat industry on the environmental and human health. However, the treatment of this type of hazardous waste results in a large consumption of water and the production of highly loaded wastewater. In order to solve these problems, this work investigates the application of pressure-driven membrane processes of ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) for the treatment and recovery of rendering plant secondary effluent after biological treatment. The rendering plant wastewater is highly loaded; thus, rendering plants must have a wastewater treatment system. The rendering plant wastewater has a high BOD/COD ratio, which makes it suitable for biological treatment. Therefore, this paper did not examine the direct treatment of rendering plant wastewater with membrane processes, but the treatment of its secondary effluent after biological treatment. So, the efficiency of commercial flat sheet (MW, GM, CQ, GK, PT, PU, and PW) and hollow fiber UF membranes (ZW-1) and NF (NF270, NF, and NF90) and RO (XLE) flat sheet membranes were examined. The process efficiency was evaluated based on the following parameters: electrical conductivity (κ), turbidity, carbon content (total carbon – TC, inorganic carbon – IC and dissolved organic carbon – DOC), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), anions (Cl-, F-, NO2-, NO3-, PO43-, SO42-, and Br-) and cations (Na+, Ca2+, Mg2+, K+, NH4+, and Li+), and pH. UF membranes showed a similar separation effect, which included the effective reduction of turbidity and COD and the retention of DOC, but slightly worse nitrogen removal, as well as poor salt retention. On the other hand, NF and RO membranes retained nitrogen and dissolved salts significantly better, but their performance varied significantly depending on the membrane used. Pristine and fouled membranes were characterized with Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM). Membrane fouling was modeled using modified Hermia models for the four fouling mechanisms (complete, partial, and internal pore blocking and cake formation). The hydrophilicity of the membrane surface was characterized by measuring the contact angle of water. In this work, the pretreatment of secondary effluent after biological treatment with sand filtration and a combined pretreatment of coagulation and sand filtration were evaluated as a strategy for membrane fouling mitigation of UF and NF membranes and UF as pretreatment for fouling mitigation of NF and RO membranes. Sand filtration was not successful in reducing the fouling of UF and NF membranes. Coagulation was optimized for the most suitable coagulation agent (from those tested: FeCl3, Al2(SO4)3, Aquaklar A (10% Al2O3), and Aquaklar C (18% Al2O3) and optimal working conditions (pH value and concentration of the coagulation agent). The most suitable coagulation agent was FeCl3 at pH value of 5.87 at a concentration of 10 mg Fe3+ L^-1. The combination of coagulation at optimal conditions and subsequent sand filtration resulted in a successful fouling reduction (50 – 95% lower flux decline) for UF membranes and 76 – 95% for NF membranes. On the other hand, using MW UF flat sheet membrane as pretreatment for NF and RO reduced the flux decline by 50 – 72%, while with ZW-1 hollow fiber UF membrane the reduction was 2 – 67%. The cleaning of UF membranes fouled during the treatment of rendering plant secondary effluent with two chemical agents (PermaClean 99 - alkaline agent and PermaClean 77 - acidic agent) was investigated. The alkaline agent performed significantly better at removing foulants from the membrane surface. In addition, the effect of the applied pressure during the treatment of secondary effluent on the irreversibility of membrane fouling and the influence of the temperature of the alkaline chemical agent on the cleaning efficiency was examined. Lastly, a conceptual solution for the reuse of secondary effluent by POZN membrane processes was proposed. The solution included the pretreatment of the secondary effluent by coagulation under determined optimal conditions, sand filtration to remove residual floccules. After the pretreatment, three stages of UF with MW membrane followed, and at the end, the UF permeate is treated with XLE membrane. The RO (XLE) permeate would be used for steam production, while the RO retentate would be used for washing factory floors and vehicles. The implementation of the proposed solution could recover 47.3% of secondary effluent (11.1% as boiler water and 36.2% for washing purposes)