SB 5.26.23:

ye tv iha vai vrishali-patayo nashta-saucacara-niyamas tyakta-lajjah pasu-caryam caranti te capi pretya puya-vin-mutra-sleshma-mala-purnarnave nipatanti tad evatibibhatsitam asnanti

The shameless husbands of lowborn sudra women live exactly like animals, and therefore they have no good behavior, cleanliness or regulated life. After death, such persons are thrown into the hell called Puyoda, where they are put into an ocean filled with pus, stool, urine, mucus, saliva and similar things. Sudras who could not improve themselves fall into that ocean and are forced to eat those disgusting things.

PURPORT

Srila Narottama dasa Thakura has sung,

karma-kanda, jnana-kanda, kevala vishera banda,
amrita baliya yeba khaya
nana yoni sada phire, kadarya bhakshana kare,
tara janma adah-pate yaya

He says that persons following the paths of karma-kanda and jnana-kanda (fruitive activities and speculative thinking) are missing the opportunities for human birth and gliding down into the cycle of birth and death. Thus there is always the chance that he may be put into the Puyoda Naraka, the hell named Puyoda, where one is forced to eat stool, urine, pus, mucus, saliva and other abominable things. It is significant that this verse is spoken especially about sudras. If one is born a sudra, he must continually return to the ocean of Puyoda to eat horrible things. Thus even a born sudra is expected to become a brahmana; that is the meaning of human life. Everyone should improve himself.

Krishna says in Bhagavad-gita (4.13), catur-varnyam maya srishtam guna-karma-vibhagasah: “According to the three modes of material nature and the work ascribed to them, four divisions of human society were created by Me.” Even if one is by qualification a sudra, he must try to improve his position and become a brahmana. No one should try to check a person, no matter what his present position is, from coming to the platform of a brahmana or a Vaishnava. Actually, one must come to the platform of a Vaishnava. Then he automatically becomes a brahmana. This can be done only if the Krishna consciousness movement is spread, for we are trying to elevate everyone to the platform of Vaishnava. As Krishna says in Bhagavad-gita (18.66), sarva-dharman parityajya mam ekam saranam vraja: “Abandon all other duties and simply surrender unto Me.” One must give up the occupational duties of a sudra, kshatriya or vaisya and adopt the occupational duties of a Vaishnava, which include the activities of a brahmana.

Krishna explains this in Bhagavad-gita (9.32):

mam hi partha vyapasritya
ye ‘pi syuh papa-yonayah
striyo vaisyas tatha sudras
te ‘pi yanti param gatim

“O son of Pritha, those who take shelter in Me, though they be of lower birth – women, vaisyas [mercahnts], as well as sudras [workers] – can approach the supreme destination.” Human life is specifically meant for going back home, back to Godhead. That facility should be given to everyone, whether one be a sudra, a vaisya, a woman or a kshatriya. This is the purpose of the Krishna consciousness movement. However, if one is satisfied to remain a sudra, he must suffer as described in this verse: tad evatibibhatsitam asnanti.

SB 5.17.11 purport:

Brahmanas are supposed to acquire six kinds of auspicious qualifications: they become very learned scholars (pathana) and very qualified teachers (pathana); they become expert in worshiping the Lord or the demigods (yajana), and they teach others how to execute this worship (yajana); they qualify themselves as bona fide persons to receive alms from others (pratigraha), and they distribute the wealth in charity (dana).