The Dairy Wastewater Treatment Biology Essay

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This paper aims to identify the effluent properties of wastewater produced in dairy product processing plants, and elucidate the typical methodology for treated wastewater. It will then seek to identify some emerging dairy wastewater treatment methodologies, and examine the use and effectiveness of these technologies through various implementations. This report will also highlight various environmental, operational (OPEX) and capital (CAPEX) expenditure impacts of these emerging treatment methods.

2 Composition of dairy wastewater effluent

The composition of dairy wastewater effluent is diverse, and deeply dependent on the type of product being produced within the dairy processing plant. Thus, the treatment mechanisms selected for the processing of associated effluent must consider the effluent constituents for the product type and volume produced within the processing plant. Effluent constituents are summarised in Table , which offers an indicative range and average for each constituent (ANZECC and ARMCANZ, 1999).

Component

Range (mg/L)

Average (mg/L)

Suspended Solids (SS)

24-5700

-

Total Solids (TS)

135-8500

2397

Biochemical Oxygen Demand (BOD)

450-4790

1885

Protein

210-560

350

Fat

35-500

209

Carbohydrate

252-931

522

Nitrogen (N)

15-180

76

Phosphorus (P)

11-160

50

Sodium (Na)

60-807

-

Chloride (Cl)

48-469

279

Calcium (Ca)

57-112

-

Magnesium (Mg)

25-49

-

Potassium (K)

11-160

67

 

Range

Average

Temperature (oC)

12-49

24

pH

5.3-9.4

7.1

Table : Indicative dairy effluent characteristics (ANZECC and ARMCANZ, 1999)

Of note in these figures are the high levels of SS, BOD, protein, fat, and carbohydrates. N and P constituents may have an environmental impact on receiving waters as a nutrient source for eutrophication processes. Thus, a wastewater treatment plant for dairy effluent should consider the reduction of each of these constituents (Britz, et al., 2006; ANZECC and ARMCANZ, 1999; Knowlton, et al., 2006).

3 Traditional treatment methods

Best practice in the processing of dairy processing wastewater is separately defined according to the composition of the wastewater which, as noted in ANZECC and ARMCANZ, is specific to the dairy processing plant (ANZECC and ARMCANZ, 1999). The Victorian EPA defines these separate processes, and asserts that the choice of treatment should be driven by the saline content of the effluent, and BOD content of the effluent - provided as high, moderate, and low BOD concentrations (EPA Victoria, 1997). The unit process flow is described as including screening, flow equalisation and pH correction, anaerobic digestion (if required), FOG and SS removal, and aerobic waste pondage (EPA Victoria, 1997). Low BOD containing effluent is recommended to be treated by micro-straining and aerobic waste pond storage (EPA Victoria, 1997).

Rusten et al treatment plant design for effluent from a cheese factory. The process is presented in Figure : Dairy Wastewater Treatment flowsheet - adapted from (Rusten, et al., 1996) below (Rusten, et al., 1996).

Influent

Screen

Grit

 

Aeration Basin

Trickling Filter x 2

Coag. Floc.

Clarifier

Effluent

Aeration Basin

WWTP

Return Sludge

FeCl3

Equal. Tank

Figure : Dairy Wastewater Treatment flowsheet - adapted from (Rusten, et al., 1996)

3.1 Preliminary treatment

It can be seen in this flowsheet that the basic wastewater treatment processes utilised in this particular factory adhere to the best practice process outlined by the Victorian EPA for an average COD of 2740 mg/L, SS of 900 mg/L, and P of 29 mg/L. In this process preliminary treatment occurs via screening, which proceeds through to an aeration basin for DO loading for the primary treatment process, and finally to an equalisation tank which tackles operational flow variations (Rusten, et al., 1996). Of note, Britz et al asserts that keeping the effluent in an equalisation tank for 6 - 12 hours aid not only in flow variation control, but also has significant benefit in pH balancing (Britz, et al., 2006). Hence, the equalisation process in this scheme aids in pH balancing the effluent for the primary treatment process.

3.2 Primary and secondary treatment processes

Primary treatment consists of a two stage (series) trickle filter process. This process is described by various sources as an attached growth treatment process, which utilises a biofilm to aerobically consume various organic compounds within the wastewater (Tchobanoglous, et al., 2004; Davis, 2010). Kessler notes that a 92% BOD removal rate is achieved through the implementation of trickling filters (Kessler, 1981, cited in Britz, et al., 2006). Secondary effluent treatment is achieved through the coagulation and floculation processes, which are then sent to a teriary clarafier for floc settling. Finally, the sludge from the clarifier is stored in an aeration basin for the continued oxidation of the sludge. Final sludge treatment is executed at a municipal wastewaer treatment facility (Rusten, et al., 1996).

4 Treatment methods for the possible recycling of water and dissolved components

4.1 Reverse osmosis (RO)

Vourche et al. conducted a study that spanned 11 dairy wastewater treatment plants in France for the purpose of examining RO as a treatment process for dairy plant wastewater. The study showed that, across the 11 plants studied, RO was capable of achieving 90-95% water recovery, with effluent quality of a standard that could be reused for applications such cleaning, cooling, and heating. The study concluded that a RO process coupled with an effluent polishing process could produce water suitable for potable use in some cases (Vourch, et al., 2008).

The Murray Goulburn Co-operative has made extensive use of RO in its dairy Leitchville wastewater treatment plant. Stated figures show that 450 kL/day of water is processed via RO for the purpose of reuse within the plant itself (MGC Co. Ltd., 2008). Annualised savings are not presented, and as such OPEX/CAPEX considerations cannot be measured for this operation. It is apparent, however, that OPEX would need to be measured against energy and maintenance costs for the RO system in this plant versus water costs.

4.2 Sequencing batch reactors (SBR)

The SBR process is an aerobic process which utilizes a fill and draw reactor which includes "complete mixing during the batch reaction step and where subsequent steps of aeration and clarification occur within the same tank (Tchobanoglous, et al., 2004)." The main benefit of the SBR process sludge return is to the aeration tank is not required since both aeration and settling occur within the same tank (Tchobanoglous, et al., 2004).

SBR is reportedly cost effective in primary and secondary treatment applications, and is able to operate with COD removal rates of between 91-97% (Ergolu et al., 1992 and Samkutty et al., 1996 cited in Britz, et al., 2006).

In Australia, the Murray Goulburn Co-operative runs its Leongatha wastewater treatment plant utilising SBR as a tertiary treatment process. A flowsheet derived from the MGC environmental report is produced below (MGC Co. Ltd., 2008).

Influent

Screen

Grit

 

Equal. Tank

Anaerobic Digester

SBR

Effluent

Return Sludge

DAF

Holding Tank

Sludge Tank

Figure : MGC Leongatha dairy wasterwater treatment process

In this scheme, the preliminary treatment of raw influent is screened and detained in a 1000 m3 equalization tank. Under shock COD or pH conditions influent is diverted to a 500 m3 holding tank and is slowly cycled back into the equalization tank. Primary treatment consists of lowering the influent pH to 4.2, and subjecting it to a Dissolved Air Flotation (DAF) process which is responsible for FOG and protein concentration reduction. Secondary treatment occurs through a 30 million litre anaerobic digester for particle hydrolysis, and microbial digestion which produces methane and carbon dioxide. Tertiary treatment occurs via two 10,000 m3 SBRs to polish the wastewater. The SBR produces carbon dioxide and sludge, of which the sludge is cycled back to the anaerobic digester for further processing. Sludge is removed from the anaerobic digester in six monthly intervals. Sludge is, at this point, utilized as fertilizer in local farms (MGC Co. Ltd., 2008).

OPEX is not generally considered for these projects, however, MGC has stated that its CAPEX for the Leongatha plant is $20 million, which is subsidized by grants from the Victorian State Government. This may prove to be prohibitive for smaller wastewater treatment plants, however, the wastewater produced is of a quality that meets Victorian EPA guidelines, and as such effluent may be discharged into local waterways (MGC Co. Ltd., 2008).

4.3 Upflow anaerobic sludge blanket (UASB) reactors

The UASB reactor process utilizes the settling properties of granulated sludge to achieve effluent component removal. In this process, influent flows upward from the bottom of the reactor (upflow) through a sludge blanket located at the bottom of the reactor. Once influent passes the sludge blanket, sludge, effluent, and biogas are separated (Tchobanoglous, et al., 2004). Low hydraulic retention times are achievable (as low as 4 hours), and sludge retention times are significantly long (as long as 100 days) (Britz, et al., 2006).

A notable byproduct of the UASB reactor is biogas output. The biogas byproduct mainly consists of methane (reported up to 70%), which may be harnessed to power elements of the wastewater treatment plant. The South Caernarvon Creameries processing plant in the UK utilized UASB to produce a full power source for the whole plant. Effluent quality was high enough for disposal to local waterways. OPEX expenditure was reduced by £30,000 to nil, and power costs were reduced by £169,000 pounds measure in terms of oil and electricity savings. The literature notes that these figures were dated 1984, and as such should be inflated to represent modern day cost savings (Anon, 1984 cited in Britz, et al., 2006).

A modern example is presented through Borculo Whey Products of the Netherlands who have utilized the UASB process to upgrade an existing plant. Reported savings include 930,750 kW hr/yr reduced energy demand due to aeration process reductions, and reduced sludge treatment and transport energy in the order of 25,000 MW hr/yr. Methane reclaimed from the UASB reactor accounts for 700,000 m3/yr. The CAPEX for the upgrade (without a detailed description of the upgrade) was stated as US$1.8 million, with an annualized OPEX saving of US$508,000 per year (UNEP, 2004).

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