ERT 319 Industrial Waste Treatment Semester /2013 Huzairy Hassan School of Bioprocess Engineering UniMAP

Solids Waste Treatment & Disposal

Solids Sources, Characteristics, & Quantities Sources - Vary according to the type of plant and its method of operation

Characteristics -Vary depending on the origin of the solids, the amount of aging that has taken place, and the type of processing to which they have been subjected.

Quantities

-The volume of sludge depends mainly on its water content and only slightly on the character of the solid matter. - A 10 % sludge, for ex, contains 90 % water by weight. -If the solid matter is composed of fixed (mineral) solids and volatile (organic) solids, the specific gravity of all of the solid matter is: -Volume of sludge:

Example 14-1 Volume of Untreated and Digested Sludge Determine the liquid volume before and after digestion and the percent reduction for 500 kg (dry basis) of primary sludge with the following characteristics: PrimaryDigested Solids, % Volatile matter, % Specific gravity of fixed solids Specific gravity of volatile solids ≈ (destroyed) 2.5 ≈ 1.0

Solids Processing Flow Diagrams

STABILIZATION -- DIGESTIONS --

Stabilization process of sludges for  volume reduction, production of usable gas (methane), and improving the dewaterability of sludge Solids and biosolids (sludge produced from primary or secondary treatment) are stabilized to: - reduce pathogens - eliminate offensive odors - inhibit, reduce, or eliminate the potential for putrefaction (decay, breakdown, corrosion).

Alkaline Stabilization -A method to eliminate nuisance conditions in sludge is through the use of alkaline material to render the sludge unsuitable for the survival of microorganisms. -In alkaline stabilization process, lime is added to untreated sludge in sufficient quantity to raise the pH to 12 or higher. -When the pH is high:  creates environment that stops or substantially retards the microbial reactions that can otherwise lead to odor production and vector attraction.  the sludge will not putrefy, create odors, or pose a health hazard so long as the pH is maintained at this level.  also inactivate virus, bacteria and other microbes present.

Application of Alkaline Stabilization Processes: 1)Lime Pre-treatment - addition of lime to sludge prior to dewatering 2) Lime Post-treatment - addition of lime to sludge after dewatering 3) Advanced Alkaline Stabilization

ANAEROBIC DIGESTION Involves the decomposition of organic and inorganic matter (principally sulfate) in the absence of molecular oxygen. Major application: in the stabilization of concentrated sludge produced from the treatment of municipal and industrial wastewater. Can produce sufficient digester gas (methane) to fulfill the energy (heat & electricity) needs.

Design Factors of Anaerobic Digester. 1)Solids & Hydraulic Retention Times - Solids retention time (SRT)  average time the solids are held in the digestion process (Mass / day) - Hydraulic retention time, τ  average time the liquid is held in the digestion process (Volume /day) - Digestion systems without recycle  SRT = τ - Hydrolysis, fermentation and methanogenesis (3- reactions in anaerobic process) are directly related to SRT - If the SRT < min. SRT in each reaction, bacteria cannot grow rapidly enough  digestion fails

2) Temperature - determines rate of digestion - rates of hydrolysis & methane formation - mostly operates in mesophilic temperature - between 30-38⁰C (or thermophilic T  ⁰C) - important since bacteria of methane-formers are sensitive to temperature changes (ex: ΔT > 1 ⁰C/d affect performance, thus ΔT < 0.5 ⁰C/d is recommended)

3) Alkalinity - Ca, Mg and ammonium bicarbonates are examples of buffering substances found in a digester. - Digestion process produces ammonium bicarbonate from the breakdown of protein in the raw sludge feed (others found in feed sludge) - The concentration of alkalinity proportional to the solids feed concentration. - Alkalinity mostly in well-established digesters  2000 – 5000 mg/L. - The principal consumer of alkalinity is CO 2, i.e.,  CO 2 is produced in fermentation and methanogenesis phases. Due to partial pressure of gas in a digester, the CO 2 solubilizes and forms carbonic acid, which consumes alkalinity. - Therefore, CO 2 concentration is reflective of the alkalinity requirements.

Single-stage High Rate Digestion  Heating, auxiliary mixing, uniform feeding, and thickening of feed stream -The sludge is mixed by gas recirculation, pumping, or draft- tube mixers (separation of scum & supernatant does not take place), and the sludge is heated to achieve optimum digestion rates. -With fixed roofs or floating covers

Two-stage Digestion -a high-rate digester is coupled in series with a second tank. -1 st tank: used for digestion, heated & equipped with mixing facilities -2 nd tank: unheated, used for storage -This type of digester is seldom used because of expense of large tank building and negligible benefit of 2 nd tank.

Example 14-5 Estimating Single-stage, High Rate Digester Volume and Performance Estimate the size of digester required to treat the sludge from a primary treatment plant designed to treat m 3 /d of wastewater. Check the volumetric loading, and estimate the amount of gas produced per capita. For the wastewater to be treated, it has been found that the quantity of dry volatile solids and biodegradable COD removed is 0.15 kg/m 3 and 0.14 kg/m 3, respectively. Assume that the sludge contains about 95% moisture and has a specific gravity of 1.02.

Other pertinent design assumptions are as follows: 1. The hydraulic regime of the reactor is complete-mix 2. SRT = τ = 10 days at 35 ⁰C 3. Efficiency of waste utilization (solids conversion) E = The sludge contains adequate nitrogen and phosphorus for biological growth. 6. Constants are for a temperature of 35 ⁰C. 7. Digester gas is 65 % methane. 8. Volume of methane produced: 1359 m 3 /d

Aerobic Digestion To meet Processes to Significantly Reduce Pathogens (PSRP) requirement (see Table 14-9, 14-10, & 14-12, pg ) Used to treat: waste-activated sludge only mixtures of waste-activated sludge or trickling filter sludge and primary sludge Or waste sludge from extended aeration plants Plant capacity may up to 2 m 3 /s

Advantages compared to anaerobic digestion: i- volatile solids reduction in a well-operated aerobic digester is approximately equal to that obtained anaerobically ii- lower BOD concentrations in supernatant liquor iii- production of an odorless, humuslike, biologically stable end product iv- recovery of more of the basic fertilizer values in the sludge v- operation is relatively easy vi- lower capital cost vii- suitability for digesting nutrient-rich biosolids

Disadvantages of aerobic digestion:  High power cost is associated with supplying the required oxygen  Digested biosolids produced have poorer mechanical dewatering characteristics  The process is affected significantly by temperature, location, tank geometry, concentration of feed solids, type of mixing/aeration device, and type of tank material  Useful byproduct such as methane is not recovered.

Eqs – 14-18: -conversion of organic nitrogen to nitrate results in an increase in the concentration of H 2 ions  decrease pH, if sufficient buffering capacity is not available. -About 7 kg of alkalinity (as CaCO 3 ) are destroyed per each kg of ammonia oxidized.  about 50 % of the alkalinity consumed by nitrification can be recovered by denitrification. Eq  Eq

Figure Volatile solids reduction in an aerobic digester as a function of digester liquid temperature and digester sludge age

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