3.9.1 Microbial Cultivation and Acclimation in Bioreactor

All microorganisms was vanished since the BA was generated in incinerator with high temperature of 800–1200°C. For a biodegradation of livestock wastewater by BA, the proportion of livestock at the influent gradually increased to cultivate the microbes in bioreactor and facilitated microbes to adapt to the final organics’ load (Table 3.9 and Fig. 3.19). Finally, over 75% COD removal were achieved. It was noted that the duration of cultivation and acclimation processes was longer than general methods because the microbes were totally destroyed in incinerator and more time for microbes to acclimate within the ‘clean’ carriers were required.

3.9.2 Operation of Bottom Ash Bioreactor

The shapes of BA from municipal solid waste incinerator presented as honeycomb with irregular shape, and those particles with various-sized holes were composed of several inorganic mineral substrates. Once the reactor started to accept wastewater, the particles in reactor bed would have some position shift, then redistributed in reactor bed. The bed would have higher bulk density, and lower porosity. Finally, the bioreactor would reach to a relative balance phase. Meanwhile, the organics removal in BA bioreactor was composed of adsorption and decomposition processes, and most of organics were just remained in BA fillings instead of being totally decomposed.

Based on the two facts described above, the bioreactor should suspend the influent of livestock wastewater after a certain period of time to dry and wet the bioreactor bed. At the same time, microbes had more time to decompose the accumulated organics in reactor bed. Obviously, there existed a balance between the organics’ accumulation and decomposing, thus a period of idle time of reactor bed would truly benefit to the organics’ decomposition ability of microbes.

Hence, this BA bioreactor belongs to one kind of nature purification system with honeycombed particles. The slag bioreactor has the same operation mode with general purification systems, i.e., artificial soil filter bed, biological sand filter bed, natural soil quickly infiltration treatment system, which is the alternate operation of influent and dry and wet in the bioreactor beds.

The ratio of the time of influent introduction and dry bed was marked as the ratio of wet and dry (W/D), named as R. The operation cycle depended on the duration time of influent and dry bed, and the function relation is showed as Eq. (3.1). Obviously, under the premise of the constant R, the operation cycle was in proportion to the influent introduction time.

$\text{Operation}\text{cycle}(\text{d})=(1+1/\text{R})\times \text{influent}\text{time}(\text{h})÷24$ (3.1)

(3.1)

With stable operation under different conditions, the influent time, the ratio of W/D will be evaluated for the effects on organics removal efficiency and nitrogen and phosphorus removal in livestock wastewater.

The experiments were operated under nine groups of trials with different influent time as well as the ratio of W/D in bioreactors. In order to ensure reliable data, each set of operating conditions at least steady running more than 2 weeks, and the results obtained in the steady state were used for calculation for the average value. The influent rate of each group was maintained as 0.094 cm/min, i.e., 1 L/h.

3.9.3 Removal of COD from Wastewater Under Different Conditions

The removal rate of COD of livestock wastewater in bioreactor under different operation conditions is shown in Table 3.10. Removal of COD decreased with an increase of W/D ratio and influent duration at a W/D ratio, with a COD removal from 76.3 to 92.5%. Under the conditions of 8 h influent duration and 1:5 W/D ratio, 8 h influent duration and 1:8 W/D ratio, and 6 h influent duration and 1:5 W/D ratio, the COD concentration of effluents were lower than 100 mg/L. The optimum effluent quality can be obtained at 4 h influent duration and the 1:11 W/D ratio.

This system showed a poor performance in TN removal, at only 16.7–26.7% (Tables 3.11 and 3.12). However, the BA Bioreactor system had a good removal effect on the NH₃-N by transforming it into nitrate-N and nitrite-N, making the removal rate range from 68.8 to 98.5%.

Conventional biological removal of nitrogen involves two processes, nitrification and denitrification. During the wastewater supply for the system, NH₃-N was transferred to solid phase from liquid phase through ionic-exchange and physical adsorption. Once the wastewater supply was stopped, the nitrification reaction occurred in the presence of nitrobacteria and dissolved oxygen. Sequentially, the denitrification reactions happened on the next circle of wastewater supply with the anoxic environments, making NO₃⁻-N and NO₂⁻-N from nitrification reaction utilized by denitrifying bacteria. The best performance can be got for TN removal under the condition of the 8-h wastewater supply interval and 1:5 W/D ratio, while the optimum condition was 4-h wastewater supply intervals and 1:11 W/D for NH₃-N removal.

3.9.4 TP Removal from Wastewater Under Different Conditions

Total Phosphorus (TP) removal under a variety of operating conditions is shown in Table 3.13. It can be seen that the bioreactor had a distinctive ability to remove TP, depending on the functions of BA fillers such as adsorption, chemical conversion, and microbe assimilation. The phosphorus accumulated in the reactor by adsorption and chemical conversion can be used to synthesize cellular materials by microbe under aerobic conditions while being released under anaerobic conditions. A shorter wastewater supply interval and lower W/D ratio can help maintain the aerobic environment and phosphorus absorption, and the best performance could be got for TP removal under the condition of 4-h wastewater supply and 1:5 W/D ratio, and an effluent concentrations below 1 mg P/L can be obtained.

3.9.5 Hydraulic Loading Rate of Bioreactor

The hydraulic loading (or hydraulic loading rate) means the amount of wastewater that can be treated by the reaction bed per unit area per day in units of m³/m² d. The hydraulic loading rate is an important indicator for system design. The test was carried out with optimum wastewater allocation 4 h wastewater supply interval and 1:8 W/D ratio. The BA bioreactor was run at different hydraulic loads so as to obtain the appropriate results, as listed in Table 3.14.

The results are presented at Tables 3.15–3.17. The removal rates of COD, NH₃-N were gradually decreased with increasing hydraulic load. In order to control the water quality reaching II-class criteria specified in Integrated Wastewater Discharge Standard, the maximum hydraulic load was set at 0.254 cm/min (2 L/h of influent). Consequently, the removal efficiency of COD, NH₃-N, and TP were 85.2, 94.2, and 98.6%, respectively.

Adsorption and biodegradation are the main mechanism for contaminants removal, and the biodegradation process predominates. The BA bioreactor is of excellent capacity of phosphorous removal, and COD removal is pretty effective. Meanwhile, nitrification for wastewater is also reliable with appropriate hydraulic load and W/D ratio. Basically, contaminant removal of the bioreactor can be simplified to two stages: Rapid physical and chemical adsorption, slow biodegradation, corresponding to wastewater introduction and drying, respectively.

3.10.1 Mechanical Separation of Bottom Ash

One of the main purposes of mechanical separation of BA is to separate and collect the recyclable wastes or those against recycling for utilization and disposal, respectively. For example, those scrap metals including steel, iron, aluminum, and copper can be recycled, while those incombustible wastes such as ceramic pieces and glasses can be transported to the landfills for disposal.

The introduction of technological process is based on a waste incineration power plant in Foshan city, Guangdong province, China. The designed municipal solid waste (MSW) disposal capacity is 700 t/day, with the BA production of 140–210 t/day. A series of technologies including crushing, sieving, and sorting are applied and good results are obtained. The amount of separated wastes for scrap metal and MSW residue is 4–5 and 40–50 t, respectively, and the rest are BA. The BA has a stable characteristic and uniform size distribution, which can meet the standard for utilization. The process flow diagram of mechanical separation of BA of municipal solid waste incinerator is shown in Fig. 3.20.

BA generated from municipal solid waste incinerator is transported by transport vehicles to the storage yard before mechanical separation. The BA will be deposited in the yard for 2–3 d, where the moisture content can be removed and the BA can be dried primarily. With the size of 150×150 mm, the coarse mesh in feeder machine can entrap the bulky waste from the BA. Then the BA drops into trommel screen with the size of 50 mm. For those residue size more than 50 mm, which mainly are stone, slag, and iron, etc., are treated separately. For example, stones, and slags are conveyed to crusher machine. Meanwhile, the textiles and plastics can be sorted manually during the conveying process, and irons can be separated by magnetic separator for the first time. For residue size less than 50 mm, before flowing into the jigger machine under the action of water, the waste are separated and crushed by magnetic separator and crusher machine, respectively. In the jigger machine, with the action of aqueous medium, nonferrous metal particle groups and BA particle groups are separated due to the difference of density. Then the shaker machine can separate the heavy recyclables such as aluminum and copper. Meanwhile, the light products flow into the trommel screen with the size of 25 mm, then the oversize products including textiles and plastics and the uniform-sized BA are separated.

The equipment in mechanical separation process of BA mainly consist of trommel screens with different size, belt conveyors, crushing machines, magnetic machine, jigger machine, shakers, circulating water pumps, cleaning agitators, and different vehicles.

3.10.2 Magnetic Separation and Reselection of Iron Resources from Recycling Bottom Ash

The major components of BA are FeO, Fe₂O₃, Fe₃O₄, etc. The iron materials can be recycled through sorting or magnetic separation, and the high grade iron ore can be used as the raw materials for steel production. Therefore, it is of great significance to recycle iron resources form in BA in order to reduce pollution and maximize the utilization of resources. Magnetic separation of iron from BA is significantly influenced by the crushing particle size. It is showed that the recovery rate of metal iron is increased with the decrease of crushing particle size, when the machine crushing process is applied to treat BA. For instance, the recovery rate of metal iron were 6.4, 7.6, and 15% when the crushing particle size were 300–100, 100–80, and 70–25 mm, respectively.

In China, amount of BA is presented with waste incineration, of which the iron compound should be separated and recycled in view of sustainable development and environmental protection. Currently, the magnetic separation process, which has the advantages of high efficiency and low cost, is applied widely to the recovery of iron compound in BA. The process flow diagram of magnetic separation of iron compound is shown in Fig. 3.21.

1. Magnetization roasting
In magnetic separation process, the BA should be magnetized reduction roasting firstly. The magnetized reduction roasting process can change the magnetism of iron material from weak to strong at a certain temperature in reducing atmosphere, which will bring weak magnet separator to a broader field. The reductant mainly includes carbon, coal, CO, and other reducing gas. The reactions are shown as follows:

$3\text{FeO}+\text{C}\to {\text{2Fe}}_{\text{3}}{\text{O}}_{\text{4}}+\text{CO}$

$\text{C}+\text{CO}\to 2{\text{CO}}_2$

$\text{C}+\text{CO}\to 2\text{CO}$

$3{\text{Fe}}_2{\text{O}}_3+\text{CO}\to 2{\text{Fe}}_3{\text{O}}_4+{\text{CO}}_2$

Many parameters such as reductant dosage, temperature, time particle size of iron ore and the cooling methods may affect the separation efficiency by influencing the magnetism of product. The hematite will fail to transform to magnetite fully in the cases of lower temperature, bigger granularity of iron ore or short of reductant. However, too much reductant, excessive temperature or longer reaction time would lead to an over reduction.

2. Magnetic separation
The magnetic separation is a method which separates the magnetic materials on the basis of the magnitude of the different magnetism compounds. The magnetic separation efficiency is affected by magnetic field intensity. Generally, the stronger magnetic field intensity is, the higher efficiency it will be. However, the iron recovery will go down with the magnetic field intensity increase further. The reason for that phenomenon is that higher magnetic field intensity can bring some weak magnetic materials into products, which is detrimental to the performance of iron recovery.
Although the concentrate grade can improve to 40–50% through magnetic separation process, it still needs a further treatment to ensure a much higher grade of refined iron mine. For example, the high gradient magnetic separation machine with the performance of the high field strength (1.8–1.9 T) and high gradient, has been applied to the separation of hematite, limonite, and manganese ore, and the treatment capacity was 8–10 t/h.

3. Flotation separation
The purity of refined iron mine is still low after magnetized reduction roasting process so that a combined process technology is needed to improve the iron recovery. Flotation, also known as foam flotation, can separate materials on the basis of different surface physical chemical properties among materials. The floatability of materials depends on their affinity for water rather than their density.
Flotation reagents play an imperative role in the modern flotation process. The floatability of materials can be changed by flotation reagents so that materials can attach to bubbles selectively to meet the target of separation. Dodecylamine is used as capture reagent in inverse flotation process, and the operating rate can reach up to 60% or more through this process.

4. Gravity separation
The combination of gravity separation and magnetic separation becomes a popular technology in iron recycling from BA. Gravity separation, based on the density or particle size difference among various minerals, classifies minerals according to their density (or particle size) under mechanical force. The mixed concentrate from magnetic separation is ground finely and then enter into centrifugal concentrator with the reselection of 400 r/min and washing water of 2400 mL/min. Through above processes, the operating rate can reach up to 60%.

Generally, the BA should be classified into narrower ranges first in order to make sure that materials are separated strictly according to their density and to improve the recovery of iron.

3.10.3 Pretreatment and Procedure of Unburned Brick Production

Approximately 80% of the ash and slag are attributed to BA of MSW incineration process, and the main physical components of which are hard and of a certain strength. Because of the low content of organic matter and the good firmness of the BA, it meets the requirements of raw material for silicate brick production. Even though the BA contains some heavy metals, but according to the codes of China, it is still classified as a kind of general waste, and can be utilized as the main raw material for the production of unburned brick. For the lack of volcano activity, the cement, lime, gypsum, and chemical activator should be used as addition materials for producing bricks.

The equipment for producing unburned bricks can be classified into four categories of pretreatment equipment, mixing equipment, molding equipment, and curing equipment, including multilayer roller screen, square hole screen, crusher, magnetic separator, eddy current separator, ball mill, twin-shaft mixer, hydraulic brick-pressing machine, and sprayer.

Silicon–aluminum materials, mainly consisting of silica and alumina, are the raw material of the production of unburned and nonautoclaved brick. The main reaction in the producing process of unburned brick is as followed:

${\text{SiO}}_2+x\text{Ca}{(\text{OH})}_2+(n-x){\text{H}}_2\text{O}=x\text{CaO}·{\text{SiO}}_2·n{\text{H}}_2\text{O}$

${\text{Al}}_2{\text{O}}_3+x{\text{Ca(OH)}}_2+(n-x){\text{H}}_2\text{O}=x\text{CaO}·{\text{Al}}_2{\text{O}}_3·n{\text{H}}_2\text{O}$

Calcium silicate crystallization is formed for the reaction between the silicon–aluminum materials and hydroxide calcium produced from cement hydration.

Manufacturing technology of unburned brick with BA of MSW incineration include different processes such as pretreatment, screening and crushing, metals separation, preparation of powder, pressing molding, first quality test, natural curing, second quality test, and degraded products reuse. The whole process is presented in Fig. 3.22.

Due to water quenching, the moisture of BA is relative high and difficult to be broken. Furthermore, the wet BA stinks and must be dried and deodorized. Drying and deodorization are performed at the same time, and the foul smell of BA can be greatly reduced during the tile drying process. Then, the BA will be sifted through 3 mm sieve after the pretreatment, and the product with particle less than 3 mm will be reserved. After that, the BA is crushed by the crusher and ball mill, and magnetic separator and eddy current separator are used to separate the iron magnetic material and nonferrous metals from broken BA, respectively.

Next, the BA is screened to adjust the distribution of different particle sizes. The BA would be sifted over 0.15, 0.3, 0.6, and 1.18 cm sieves, respectively, and be mixed in a certain quality proportion, for instance, 0–0.15 cm accounts for 40%, 0.15–0.3 cm accounts for 30%, and 0.6–1.18 cm accounts for 30%. The BA, lime, and gypsum are added into the twin-shaft mixer according to a certain proportion, and then evenly mixed the mixture with the activator solution and cement together. During the mixing period, the water content should be controlled to be about 8–12%.

The power is prepared for compaction, and pressed by double-sides pressure using hydraulic machine. The process is completed through four stages and the pressure is about 17–23 MPa. In first stage, the preloading pressure is relative light and a portion of air is discharged. A heavy pressure is applied in the second stage, and discharging another portion of air. A final pressing is carried out in the third stage, and finally holding the pressure before the BA is molded.

The molded bricks would be tested to eliminate the degraded products and made a timely adjustment of the formula or the renovation of mold in order to reduce the consumption of raw materials and the wear and tear of machine. A sprinkling water natural curing process is needed when the molding products have a certain strength. Finally, the second quality test is carried out, graded products after curing would be on sale. While the degraded products, from first and second quality test, are crushed again to reproduce bricks.

3.10.4 Wet Treatment Process for the Recovery of Nonferrous Metals from Bottom Ash of MSW Incineration

In general, BA of MSW incineration contains economically significant levels of silver, gold, and other nonferrous metals (Table 3.18). Currently, the mineral fraction of BA can be reused as the secondary building and road material, and the nonferrous metals can be recycled by dry or wet treatment technologies. Conventional BA treatment is a dry separation process, which is always applied to recover the particles larger than 10–15 mm from the ash. The wet process can treat fine particles (down to 100 μm) to recover the nonferrous content from the BA using quantities of water.

As shown in Figure 3.23, the equipment in washing process of BA consists of control system, pretreatment system, water washing system, and separation system. Mostly, the BAs have large pieces of debris as well as some combustible contents. Those fractions of waste are useless for the recycle process and should be removed first by sending to the landfill or burning again. After a preliminary magnetic density separation (MDS), ferrous scrap can be obtained with some regular equipment. The residue from MDS continues to be crushed and the bulky parts are further removed through the wet screen where water dissolves some metal salts so that the recovery will be easily processed subsequently.

The heavy nonferrous metals are conveyed to jigging machine from the BA. The heavy particles, including metals and other fractions, are collected through pipeline. A shaking table equipped with a grit chamber is performed to separate the rest of heavy nonferrous metals. With these, almost all the metals get separated from the BA, as the aggregates can be utilized after dewatering.

Apart from water, the BA can also mixed with acid solution. Some metals like Cu and Zn are dissolved into the solution and separated from nonsoluble Pb by filtration. Then, Cu and Zn can precipitate by alkali or sulfide. By this treatment, lead and zinc cakes can be obtained. This process is similar with the hydrometallurgical method whereas the products rarely meet the high cost quality (Fig 3.23).

Both the dry and wet processes almost have the same recovery rate of ferrous metals. In China, little BA is recycled and treated by the wet process because the eddy current separation result is not good, especially for 0–5 mm copper, which has a high value in market. Therefore, the wet treatment process have a better profit if considering the product price and treatment cost (Table 3.19). Moreover, the wet process generates a great deal of wastewater and sludge, leading to the increase of operation cost for pollution control. To date, there still need some improvements and innovations for the implementation of recycling precious metals from BA.